Spain's Rafael Nadal made a winning return after eight months out with victory in his opening men's doubles match alongside Juan Monaco at the Chile Open.
But after beating Czech second seeds Frantisek Cermak and Lukas Dlouhy 6-3 6-2, he admitted he is still in pain.
"The doctors have said it's OK. There is no risk of making it worse," he said.
28 June, 2012:
Nadal loses to Lukas Rosol in second round of Wimbledon
Pulls out of an exhibition match with Novak Djokovic because of tendonitis
Withdraws from the Olympics with tendonitis in left knee
Withdraws from the US Open
Announces he has a partially torn patella tendon
Confirms he will not play in the Paris Masters or ATP World Tour Finals
Practises on court for the first time in Majorca
Pulls out of the Abu Dhabi exhibition with a stomach virus
Withdraws from the Australian Open because of the virus
5 February, 2013:
Nadal returns alongside Juan Monaco in the Vina del Mar doubles before playing his first singles match
"My knee keeps hurting. But the fact I am playing here is a thing of joy."
Nadal, 26, teamed up with Argentina's Monaco to win his first match since his dramatic second round five-set
defeat by Czech Lukas Rosol
at Wimbledon in June.
The former world number one, now ranked at five, was forced to withdraw from the Australian Open after his recovery from tendonitis in his knee
was hit by a stomach virus.
He also missed the London 2012 Olympics and the US Open.
"I am not 100%, I need some weeks," he added. "If it hurts, it hurts and we'll put up with it. I am here to play tennis, with or without pain. I'm happy to have played an official game, although it was doubles.
"It was a great feeling to play again and I'm very happy for doing it with a friend like Monaco in a great atmosphere that we had on the court.
"In doubles the effort is always less [taxing]. We will see how it goes in singles and how my physical condition is. I chose this tournament because it is the perfect time and surface."
Nadal will continue his comeback on Wednesday when he takes on Argentine qualifier Federico Delbonis in singles.