Live Commentary: Serena Williams vs. Venus Williams - as it happened

Good morning everyone and welcome to the big one - sisters Serena and Venus, in battle for the 27th time of their careers. This time it's the quarter-final of the 2015 US Open, and a 'Serena Slam' is on the line.

This fixture was scheduled to begin at 12, but after Jo-Wilfried Tsonga and Marin Cilic's five-set thriller proceedings have been put back. You can read about that game - and what a match it was - HERE while you wait for the Williams sisters.

They met here in the first prime time women's #usopen final 14 years ago. Who wins tonight? #Serena #Venus https://t.co/ghqE3YhCxa — US Open Tennis (@usopen) September 8, 2015

This is the fifth occasion that Serena and Venus have met in the US Open. Venus won in 2001 and 2005, while Serena won in 2002 and 2008.

Serena has not lost to Venus at a major tournament since Wimbledon in 2008, but the world number one admitted to reporters on Tuesday that she does not want to face her sister tonight: "She's beaten me so many times. I've taken a lot of losses off her – more than anybody. She's a player that knows how to win, knows how to beat me, and knows my weaknesses better than anyone."

Meanwhile, Venus told ESPN on Sunday that she's pleased to be in the quarter-finals with her sister: "I'm so proud of Serena, and I think she's proud of me. We inspire each other."

On her way to tonight's match, Serena defeated Madison Keys 6-3 6-3 on Sunday, while Venus impressed in a 6-2 6-1 win over Estonian Anett Kontaveit.

And this is why tonight's match is so big...

Together, #Serena & #Venus have won 115 career singles titles, 28 singles majors and have spent 268 weeks at No. 1. pic.twitter.com/K4okn4amjR — US Open Tennis (@usopen) September 8, 2015

Serena has been world number one since 2013, but her sister is now ranked at 23. The two played at Wimbledon earlier this year, where Venus was dumped out in the last 16, but you cannot write her off!

We're still waiting for Serena and Venus to take to the court at the Arthur Ashe Stadium, and as soon as they do we'll have all the action here for you on Sports Mole.

Right on cue! The sisters have arrived...

The last person to win a calendar slam was Steffi Graf in 1988. That's the prize that Serena is lining up at Flushing Meadows, but Venus could well put a halt to that tonight.

Here we go then folks! Serena vs. Venus, a classic encounter, in the quarter-finals of the US Open. It begins...

Strong start for Venus, whose service game was on point in the first game here. Serena hits two errors as Venus takes a 40-30 lead, but Serena recovers. An ace gives Venus the advantage, before a solid forehand winner wraps up the game.

Serena's serve is equally impressive, as Venus struggles to respond. The world number one wraps up the game by forcing an error from her sister - this match looks set to be tight folks.

Venus races to the lead with some aggressive play, moving into the net to go 30-0 up, before an impressive forehand winner takes her to 40. Serena responds with a neat touch shot, but Venus finishes with another strong forehand winner. Good match so far - did we expect anything less?

Brilliant tennis! Venus hits a ferocious return to take it to 15-15, before a superb rally ends with Venus mishitting a shot at the net. Serena goes 40-15 ahead as her sister hits a return wide of the court, but Venus recovers with a brilliant forehand winner, only for the younger Williams to take the game.

BREAK! Venus continues to impress with her service, hitting a powerful ace to go 30-0 up, but Serena recovers. The world number one then shows some great movement before hitting a backhand winner to move into the lead, before an error allows Venus to level up. A forced error from Serena gives her sister the advantage, but the 33-year-old again recovers with a volley at the net, before eventually breaking with a Venus error.

Serena is starting to pull away now, hitting her first ace to begin the game's scoring, before forcing an error from Venus. The 35-year-old responds well though with an impressive forehand winner, before the sisters exchange points through errors. Serena wraps it up as Venus hits the net at 30-40 down.

BREAK! Confusion on the court as Venus thinks her serve is out, but Serena hits a forehand winner with play allowed to continue. Venus responds with an ace, but her sister levels it up with some great play from the baseline. Serena takes advantage with a brilliant lob over Venus that lands well inside the court, and the world number one completes the break with a Venus error.

SET! Stunning backhand return from Serena moves her into the lead, before a forehand winner puts her 30-0 up. The world number one hits her second ace of the match, before taking the first set as Venus is forced into an error from her sister's final serve. First set to Serena - can Venus come back?

Better from Venus, as Serena struggles to respond to her service and goes 0-30 down. Venus hits a teriffic backhand winner to go 40-0, before forcing another error from her sister. Venus rolling back the years in this game! Can she keep it up?

A strong forehand winner gives Serena a 30-0 lead, but a double fault gives Venus a point. Serena responds with an ace, before forcing an error from her sister to take the game.

Venus responds well, forcing a couple of errors from Serena before completing the game with an ace. Serena did not take a single point in this one.

BREAK! Serena starts with an ace, but falls to 15-30 behind after a couple of errors. Serena recovers, and goes to 40-30 up after a couple of Venus errors, but Venus responds with a strong forehand winner. Venus takes the advantage after some impressive play from the baseline forces Serena into an error, but the 35-year-old is unable to win her first break point of the night. Venus produces an impressive return to go ahead again though, before a double fault gives her the first break. She leads 3-1 in the second set, she couldn't stop the slam could she?

A fearsome ace gives Venus the lead, before Serena is forced into three quick errors and the number 23 seed takes the game. Venus is starting to look more comfortable with every point, finding her range of shots well. Serena is starting to look a little shaky out there!

BREAK! Venus moves into a quick 0-30 with more Serena errors, but Venus hits a forehand winner out past the baseline to give Serena a point. Venus is forced into an error to level the points, but moves ahead again as Serena hits the net. Serena recovers after a lengthy rally, but a double fault gives her sister the advantage, before a Serena return goes out. Venus moves to 5-1 up in the second!

Venus slips behind with a backhanded shot at the net that hits said net, but recovers, only for Serena to go 30-15 up with an impressive forehand winner. Venus recovers again though with a strong backhand winner, before Serena hits a forehand shot beyond the reach of her sister. Venus levels by forcing an error, before her sixth ace provides a set point opportunity, only to hit a backhand shot out. Venus gains another set point opportunity, but again squanders the chance with an error. At the third time of asking Venus takes the set, with Serena hitting the net. This one will go the distance, and Venus is looking strong - game on!

That was superb from Venus, who raced to a 6-1 set win. Is a shock on the cards at Flushing Meadows?

Serena hits a delightful shot beyond Venus to go 30-0 up, before an error gives her sister a point. Venus hits a superb backhand winner to level the score, but Serena responds with a terrific ace. The world number one wraps up the game with another ace as she responds to a poor set last time out.

BREAK! Venus hits another ace to take the lead, but Serena recovers, before hitting a forehand winner to go 30-15 up. Serena follows it up with a backhand response to go further ahead, before breaking with a brilliant shot from the baseline. Serena is finding her form here at the Arthur Ashe!

Serena storms ahead with an ace, before forcing an error from Venus to go 30-0 up. Venus recovers though, and levels the scores with a superb forehand winner, before moving ahead after Serena hits an error. Serena recovers with another ace, before hitting a superb backhand shot to take the advantage. Serena holds on under pressure and moves 3-0 up in the final set.

Serena does well to adjust and keep a rally going, but ends the move with an error to go 0-30 down. Venus takes another point as Serena hits a forehand shot beyond the court, before Venus wraps the game up with a forehand winner. Venus has stopped the rot in the final set.

A double fault gives Venus the lead, but Serena follows it up by forcing an error with a powerful serve. Serena hits a backhand winner beyond her sister, before finishing with an ace. Serena is looking really strong now, and she's closing in on a victory over her older sister at the Arthur Ashe.

Serena produces a strong return to force an error from Venus, but her sister recovers. The 35-year-old then moves ahead before an error of her own levels the game. Venus hits a powerful serve to force Serena into an error, before taking the game with some smart shots. Venus is rediscovering her range now, but she needs to break Serena soon.

A Serena ace brings the score to 15-15, before a superb serve forces Venus into hitting the return out. Venus recovers thanks to a Serena error, but the world number one returns with yet another ace - this one the quickest of the match at 122mph! Serena takes the game, and she moves to just one away from a place in the semi-finals.

Venus begins the game with an ace, but slices a volley into the net to level the scores. A Serena error restores Venus' lead, before Venus emerges the stronger of the two in a lengthy rally, before taking the game with an ace. Will she be back to serve again?

GAME SET MATCH! No she won't! Serena has recovered from a second-set slip-up to defeat her sister 6-2 1-6 6-3. Venus improved in the second set, but Serena was too strong in the first and third and takes an emotional win at the Arthur Ashe Stadium. Serena goes to 40-15 up as Venus responds to a powerful serve with a forehand shot that goes beyond the baseline of her opponent. The world number one - on the verge of tears - composes herself before hitting a perfect ace to take the victory!

Serena is two wins away from a calendar slam - the first since 1988. Venus was outstanding in the second set, but Serena had a resurgent spirit in the third and made sure she would take the win this morning.

You can read how Serena defeated her sister HERE.

"It's going against your best friend and at the same time going against the greatest competitor," Serena tells ESPN after her victory. Emotions are running high at Flushing Meadows this morning.

Serena will take on the unseeded Italian Roberta Vinci in the semi-finals, and the world number one is looking very strong in her battle for another Grand Slam win.

Thanks for your company this morning, that's all we have time for with the Williams sisters, but don't leave Sports Mole as we have full live commentary of Novak Djokovic's quarter-final clash with Feliciano Lopez still to come!