Kei Nishikori was disappointed he let Tomas Berdych back into their ATP World Tour Finals meeting by losing the second set, despite ending victorious.
The eighth seed recovered from a straight-sets loss to Novak Djokovic in his Stan Smith Group opener to beat the Czech 7-5 3-6 6-3.
Nishikori was frustrated by his performance in the second set, believing he was guilty of gifting Berdych a route back into the match.
"I thought there were many good shots for me. I mean, I had a bad game in the second set at 2-1, and after that he started playing better," he said.
"That was kind of my fault that I give him a chance to come back for the match."
Nishikori will face Roger Federer in his final group match and is anticipating a great match against an opponent he says is still improving.
"It's going to be really big challenge for me to play Roger. It's always exciting and it's going to be really important match for both of us," he added.
"I have to win the next match to qualify the group. So I will try to recover well and I will get good tactics ready and try to play another good match.
"He's still improving, playing great tennis even though he's 34 years old."