Novak Djokovic says he was not feeling even close to his best after seeing a 23-match unbeaten run ended in comprehensive fashion by Roger Federer at the ATP World Tour Finals.
World number one Djokovic has enjoyed a sensational 2015 and has reached 14 successive finals prior to the season-ending event at the O2 Arena in London.
However, 17-time grand slam champion Federer was at his classy best as he made it two wins from two in the group stages with a 7-5 6-2 triumph, booking a semi-final berth in the process.
Djokovic, who is aiming for a fourth successive title in London, is now keen to quickly move on from a rare defeat.
"You have those days as well when you are not feeling your best, not even close to the best," he said. "Credit to Roger for mixing up the pace, giving me always a different ball.
"He used the slice and spin very wisely. He served very efficiently. I made a lot of unforced errors, I just handed him the win, especially in the second set.
"But, again, that's sport. Sometimes these kind of matches, these kind of days happen. The good thing is that it's a round-robin system so I still have an opportunity to reach the semis.
"In a way I want to forget about it, the way I felt and played. But, again, the day that I have between the two matches will allow me to reflect on this match with my team, work on things that didn't go the way I wanted them to go."
Djokovic trounced Kei Nishikori 6-1 6-1 in his opening fixture and the Serbian - who won three out of four grand slam titles this year - will reassess what needs to be done to return to winning ways against Tomas Berdych in his final round-robin encounter.
"I practiced well. Obviously, the opening match couldn't be better and the second match couldn't be worse," he added.
"Again, as I said, this is all part of the sport. You learn as you go along. I'll take a note. Have to look back, reflect on what I've done wrong this match, get it right for next one."