Unbeaten Saracens continued their relentless push for a Heineken Champions Cup quarter-final place by beating Cardiff Blues 26-14 at the Arms Park.
But it was an often close-run contest as Saracens' 21-game run without defeat came under serious threat for large parts of the game.
England fly-half Owen Farrell's second-half penalty double and a Jamie George try edged them home as they strengthened their grip on Pool Three.
Wing Sean Maitland scored a first-half try, with Farrell kicking 16 points, while the Blues claimed touchdowns from centre Rey Lee-Lo and full-back Dan Fish.
Fly-half Gareth Anscombe kicked both conversions, yet the Blues are facing a pool-stage exit despite pushing Saracens hard just six days after suffering a 51-25 defeat against the same opponents at Allianz Park.
Saracens will secure their place in the last-eight if they beat struggling Lyon in France next month, with that game being followed by a home encounter against Glasgow.
Saracens now head to Gallagher Premiership title rivals Exeter next Saturday, taking with them an undefeated record that stretches back to April 1 when they lost a Champions Cup quarter-final to Leinster.
Saracens, looking to back up last week's stroll against the Blues, took just nine minutes to open their account.
And it was a soft try to concede from the Blues' perspective after Saracens drove an attacking lineout and then made the most of poor defensive work out wide.
Farrell, skipper Brad Barritt and wing Alex Lewington linked seamlessly, then Maitland – who scored two tries in last Sunday's fixture – rounded Fish.
It was a poor attempt by Fish to halt Maitland, and the Scotland international sprinted over for a try that Farrell converted.
Farrell extended Saracens' lead through a 17th-minute penalty, but just when it looked as though the Blues were in deep trouble, they responded in try-scoring fashion.
After gaining an attacking five-metre scrum, Blues flanker Josh Navidi found scrum-half Lewis Jones, whose pass to Lee-Lo saw him outstrip the visiting defence for a try that Anscombe converted.
Another Farrell penalty made it 13-7, but the Blues stung their opponents with a second try as half time approached.
Anscombe had little room to work in, but he kicked cleverly towards wing Garyn Smith, whose inside ball was collected by Fish, and he surged away to score.
Anscombe's conversion put the Blues a point in front as Saracens found themselves having to answer serious questions for the first time, and they preserved that advantage at the break.
FT | Cardiff Blues 14-26 Saracens ⚫️🔴 pic.twitter.com/XoDwEWDl7E — Saracens Rugby Club (@Saracens) December 15, 2018
Farrell nudged Saracens back in front when he completed his penalty hat-trick after 55 minutes, but the Blues continued to give it everything in pursuit of ending Saracens' unbeaten record this season.
And the visitors looked rattled at times, which was underlined when lock Will Skelton received a yellow card for pulling down a maul inside his own 22, giving the Blues a temporary one-man advantage inside the final quarter.
But Saracens bossed the closing minutes as Farrell slotted another penalty, then George powered over – he was awarded the touchdown despite Blues replacement Olly Robinson appearing to get his arm underneath the ball – and Farrell's conversion sealed the deal.
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