Novak Djokovic said he was bothered by a wrist problem during the Laver Cup in London and that his lengthy absence from the Tour recently could be to blame.
The three-day tournament in London's O2 Arena was Djokovic's first event since the Serb won his 21st Grand Slam singles title at Wimbledon in early July.
The 35-year-old, who missed the North American hardcourt swing and the U.S. Open due to not being vaccinated against Covid-19, produced a dazzling performance on his return to the Tour on Saturday by winning his singles and doubles matches.
However, he lost to Canada's Felix Auger-Aliassime on the final day of competition on Sunday.
"I have been struggling with my right wrist for the last four, five days, to be honest. I have been keeping it under control," Djokovic told reporters.
"The two matches yesterday probably had an effect. Today was not easy. I couldn't serve as fast or as accurately as I would like to. That has affected the whole game."
Djokovic said qualifying for November's ATP Finals in Turin remained his goal and that he will play a tournament in Tel Aviv this week followed by an event in Kazakhstan next week and then the Paris Masters at the end of October.
His Wimbledon victory guarantees him a spot in the Tour-ending Finals if he remains ranked in the world's top 20.
Djokovic, who is currently ranked seventh, said the wrist issue could be due to a combination of factors.
"Could be not playing matches almost three months, and then conditions here are such that the balls are really big and slow," he said.
"You always have to generate a lot of wrist action and speed, which could be the case why I have been feeling soreness."