Alexander Zverev recorded his first victory over troubled world No 1 Rafael Nadal as the German made a perfect start to his ATP Finals defence.
The Spaniard suffered an abdominal problem at the Paris Masters and it appeared on this showing that he has not fully recovered for the season-ending tournament in London.
He couldn't match the power and precision shown from Zverev, who dominated in every aspect to pull off somewhat of an upset win, 6-2 6-4.
Whether Nadal will be able to hold off Novak Djokovic for the year-end top spot will be the big story this week.
US Open champion Nadal had been unable to serve because of the injury and he was unsurprisingly tested in every service game until he succumbed under mounting pressure by the German, who broke through a couple of times to take the opening set.
Indoor hard courts have never been Nadal's favourite surface and injury has often got in the way of his participation at the ATP Finals.
The 2010 and 2013 runner-up here has withdrawn ahead of the tournament six times since he first qualified in 2005 and two years ago was only able to complete one match.
Zverev, who claimed the biggest title of his career here last year, continued where he had left off in the first set by outplaying his opponent in every department.
The 22-year-old moved 2-0 up, while 19-time Grand Slam champion Nadal was being forced to dig deep after an eight-minute service game just to hold serve in the fifth game. Zverev served out the contest to seal a convincing win after one hour and 24 minutes.
Stats of the match
Zverev recorded his first victory over Nadal having lost all his previous five meetings.
Carrots and spinach for Sascha
Stef steps up
Greek sixth seed Tsitsipas had not beaten Medvedev, this year's US Open runner-up, in five previous meetings with the Russian.
But the 21-year-old found London's O2 Arena to his liking as he battled to a 7-6 (7-5) 6-4 victory in a high-quality contest.
Russian Medvedev, 23, has won more matches than any other player this year, his total of 59 victories being five more than Novak Djokovic has managed.
But he could not manufacture a single break point against the dogged Tsitsipas, who secured the only break of the match at 5-4 in the second set before serving out the win.
Stats of the match
Tsitsipas: I had goosebumps
Medvedev frustrated
Who plays who next...
Zverev takes on Tsitsipas in the second round-robin matches on Wednesday, while the pressure is on Nadal when he faces Medvedev in a rematch of their US Open final marathon, which the Spaniard won.
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