The Serb defeated the 28-year-old Swiss 6-3 6-3 on Sunday at The O2 to advance to the final of the competition to face Rafael Nadal.
"I had the feeling that I was playing good, hitting well the ball," Wawrinka told reporters. "But I think I don't have enough energy. Against him, that makes a big difference.
"I'm disappointed with myself because I had the feeling that I could have done something better, but I really think that I was not fresh enough to do something."
Wawrinka has beaten Djokovic just twice in their last 17 encounters.