Novak Djokovic opened his bid for a sixth ATP Finals title with a hard-fought win over Stefanos Tsitsipas in straight sets in the Red Group.
The Serbian was forced to dig deep against his Greek counterpart in a tense second set, but he eventually prevailed 6-4 7-6[4] in one hour and 38 minutes.
Djokovic broke Tsitsipas's serve in the opening game of the match, forcing his opponent to try to earn a break against him for the very first time in his career to stay alive.
Tsitsipas fought valiantly and proceeded to post two successive love holds on serve, but the break he longed for eluded him, having hit just seven winners compared to Djokovic's 16.
A strong forehand was a feature of Tsitsipas's game in the first set, and the world number three proved much more unforgiving on serve in the second set while Djokovic faltered.
The 21-time Grand Slam champion double-faulted twice in the fourth game of the second set to hand Tsitsipas a break point, but he soon returned with the customary winners to hold for 2-2.
Again, Tsitsipas's efforts would not lead to a break of serve, and Djokovic went 5-1 up in the tiebreaker after hitting a fine backhand which clipped the line.
Tsitsipas managed to claw the second-set tiebreaker back to 5-4, but Djokovic sealed the victory on serve to move to the top of the standings in the Red Group early doors.
© Reuters
Prior to Djokovic's victory, an all-Russian battle between Andrey Rublev and Daniil Medvedev took place in the other contest in the Red Group, and the former came up with a shock result.
Rublev came through a mammoth two-hour and 31-minute encounter with a 6-7[7] 6-3 7-6[7] win to his name, in spite of a plethora of missed opportunities in the first set.
The sixth seed brought up a huge seven set points in the first set against Medvedev but failed to convert any of them, as his compatriot somehow moved 1-0 up courtesy of a tiebreaker.
However, Rublev upped his winners count - hitting 25 across the final two sets - to clinch the second set before engaging in a mammoth 37-shot rally with Medvedev in a third-set tiebreaker, by which time he had already seen four match points pass him by.
Rublev did not let cramp affect him as he smashed a forehand volley beyond Medvedev to seal a three-set win, and he left the court with a poignant message amid Russia's invasion of Ukraine.
Back in March's Dubai Tennis Championships, Rublev wrote "No war please" on a camera, and the 25-year-old shared a similar sentiment in Turin, writing "peace" and "all we need" on the equipment.
The ATP Finals continue with the Green Group on Tuesday, as Rafael Nadal faces Felix Auger-Aliassime while Casper Ruud takes on the Spaniard's recent conqueror Taylor Fritz.
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