A straight-sets win over Novak Djokovic at the ATP World Tour Finals came as a surprise to six-time champion Roger Federer, who did not expect to beat the world number one.
Federer advanced to the semi-finals after snapping Djokovic's 23-match winning streak courtesy of a 7-5 6-2 victory in London on Tuesday.
Defending champion Djokovic had not lost at the ATP Finals since 2011, while the Serbian had won four of the past five head-to-head meetings, so Federer did not have high hopes.
"I wouldn't have picked it maybe before the tournament, you know, just because of his really good record on the indoors, the year he's had, especially with the run he's been on," Federer told reporters.
"I focused more on beating Berdych and Nishikori and let's see what happens against Novak. That shows me that I didn't expect this victory. But then again, all day I felt good about my chances. It was clear in my head how I wanted to play.
"Once the match went on, I felt like, I'm with him, I'm serving well, could play better from the baseline maybe at times, but he was doing a good job there. We were just staying with each other really.
"I was a bit disappointed he got back on me, which didn't happen in Cincinnati. It was a similar situation. But he did well to break.
"I stayed calm. At the end it was not that surprising because I was playing really well and he dropped his level towards the end."