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Dominic Thiem came out on top in a high-quality match to beat Rafael Nadal at the ATP Finals.
Nadal had two set points in the first-set tie-break but could not convert with Thiem eventually securing the win on his fourth match point.
Stefanos Tsitsipas plays Andrey Rublev in Tuesday's other match of Group London 2020 at about 20:00 GMT.
If defending champion Tsitsipas wins, Thiem will be into the semi-finals before his final match of the round-robin stage.
Nadal will play Tsitsipas on Thursday while Thiem faces Rublev.
"It was a great match from the first to last point. I was pretty lucky to get that first set after being 5-2 down in the tie-break," Thiem said.
"I had the feeling that the match was a very high level. The atmosphere would've been unreal [if fans were in the arena]. We put on a good show for those watching on television."