Dominic Thiem praised Pablo Carreno Busta for his performance as Rafael Nadal's replacement at the ATP Finals.
Thiem edged past the Spaniard 6-3 3-6 6-4 in Group Pete Sampras on Wednesday, keeping his semi-final chances alive in London.
Carreno Busta came in for Nadal, who withdrew from the event with a knee injury after his loss to David Goffin.
Thiem said the world number 10 performed well after replacing his countryman.
"It was a really, really tough match. I think he took it very good, the place of Rafa," the Austrian said in an on-court interview.
"Again, very tough three-setter and I'm very happy to be through. It got very tight at the end.
"I think I started the match very well and then when he broke me after 40-0 in the second set I lost it a little bit.
"The third set was up and down, I think it was 50-50 for both players and luckily I did the deciding break."
Thiem holds a 1-1 win-loss record heading into his final group match – a meeting with David Goffin.
The 24-year-old said he was pleased to have an opportunity to reach the last four.
"There are so many thoughts in general because only if I could win this match today I could have another final on Friday," Thiem said.
"Every single match in this great atmosphere is amazing and I'm happy that I'm still playing for the semi-finals on Friday and it's going to be a tough match again."
