Rafael Nadal played his first hardcourt match in nearly a year on Monday to end the inaugural World Tennis Day.
Nadal was beaten 7-6 (7/4) 6-4 by Juan Martin Del Potro at New York's Madison Square Garden in what was, at times, a light-hearted exhibition match.
The Spaniard will now move on to Indian Wells to play in the BNP Paribas Open - his first competitive hardcourt action since Miami last March.
Nadal won the Acapulco Open on Sunday, a tournament which was played on clay.
Looking ahead to testing his knee fully on the hardcourts which are believed to have heaped pressure on the joint, Nadal said: "I really hope that the knee will go well. I don't know how well but the knee will give me the chance to compete.
"I know the knee isn't 100 per cent better yet, but if the knee is now well to be competitive, to run with no limitations or limitations on my movements, that's a lot."
In a women's match at the same BNP Paribas Showdown event in New York, world number one Serena Williams defeated Victoria Azarenka 6-4 6-3.
Azarenka will now move onto Indian Wells to defend her BNP Paribas Open title.
Britons Heather Watson and Laura Robson will begin their campaigns against Irina-Camelia Begu and Sofia Arvidsson respectively. The latter-named Swede left Robson in tears last April when beating her in a Fed Cup match.
World Tennis Day was set up to promote the game, with a particular focus on increasing participation among young people, and the first-ever such event saw more than 40 countries take part.