Nadal withdrew hours before the start of his semifinal Friday at the Sony Ericsson Open because of a left knee injury that bothered him in recent matches.
With his withdrawal, 2009 champion Andy Murray advanced to Sunday's final. His opponent will be top-ranked Novak Djokovic, who advanced by beating Juan Monaco 6-0, 7-6 (5).
Nadal said his knee has been getting worse and extensive treatment Thursday failed to help. He said he'll see a doctor soon and is optimistic he'll be out only briefly.
"It looks like it's nothing really different than happened a few times in the past," he said. "Hopefully with a few days off and with the right treatment, I will be in the right conditions to start to practice on the clay. I know what to do to get better quick, and that's what we're going to try."
The six-time French Open champion still plans to begin his clay season April 16 at the Monte Carlo Masters.
Nadal has dealt with knee problems for several years, and his left knee has bothered him since Indian Wells. He lost there to Roger Federer in the semifinals two weeks ago.
Shortly before this year's Australian Open in January, Nadal hurt his right knee while sitting in a chair at his hotel and feared he might miss the tournament. Instead he reached the final, where he lost to Djokovic in a six-hour epic.
Nadal played no tournaments in February to rest his knees. Even then, he said, his left knee bothered him some while training at home in Spain.
Knee problems prevented Nadal from playing at Wimbledon in 2009-the only major tournament he has missed since 2006. He retired with knee trouble from a quarterfinal match against Murray at the Australian Open in 2010.
"I try my best in every moment, with pain or without pain," he said. "But when I see the situation is done and I cannot, I cannot."
The 10-time Grand Slam champion had hoped to claim his first Key Biscayne title. His withdrawal left the tournament without a singles match for the afternoon session.
"I am very sorry for the fans," Nadal said. "I'm very sorry for the tournament. I'm very sorry for everybody who was ready to watch the match on the television, for television, for everybody.
"I don't have pleasure. I feel very sad to have to go out before a beautiful match for me-semifinals against Andy."
The No. 4-seeded Murray has played only seven sets in the tournament, because he also advanced to the third round with a walkover. He'll go into the final with three days' rest but perhaps a bit rusty.
"I don't really know how I'm going to feel," he said, "but I'll definitely be fresh."