Dominic Thiem became the first man to move into the last four of the ATP Finals event in London.
The Austrian beat Rafael Nadal 7-6 (9-7) 7-6 (7-4) in a high-quality match for his second victory of the season-ending event at the O2 Arena in London.
Nadal had two set points in the first-set tie-break but could not convert with Thiem eventually securing the win on his fifth match point.
Thiem's qualification was secured when Stefanos Tsitsipas beat Andrey Rublev.
Defending champion Tsitsipas survived a match point as he won 6-1 4-6 7-6 (8-6) in Tuesday's other match of Group London 2020.
Tsitsipas will play Nadal on Thursday with the winner joining Thiem in the semi-finals, while Rublev is eliminated after two defeats from two.
"It was a great match from the first to last point," said Thiem of his win over 20-time Grand Slam winner Nadal. "I was pretty lucky to get that first set after being 5-2 down in the tie-break.
"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."