Novak Djokovic was pleased to come away with a confidence-boosting win over Milos Raonic, but feels there is still room for improvement.
The Serbian world number two booked his spot in the last four of the ATP World Tour Finals with a 7-6 (8-6) 7-6 (7-5) win on Tuesday.
While Djokovic was happy to claim his victory, he said it could have been more comfortable.
"I should have done my job earlier, to be honest," he said.
"I'm not very pleased to drop my serve twice against Milos, especially the second time. I was 4-3, 30-0, then just four pretty bad unforced errors.
"Credit to him for really hanging in there, putting pressure, being aggressive, especially from the forehand. But I think I should have done better there.
"Again, all in all, two tie-breaks against a big server is a great win and great confidence boost."
Djokovic faces Frenchman Gael Monfils on Thursday, but he is already guaranteed a semi-final berth from Group Ivan Lendl.
As for his form, the 12-time grand-slam champion was mostly pleased, saying: "Other than a couple of loose service games, I thought it was a good performance.
"I should have maybe been a bit more aggressive in certain points, maybe too static in certain points. Maybe that caused a little bit passive game.
"In general, this definitely can serve as a great wind in the back for the next match, matches to come."