NEW YORK — Roger Federer couldn't figure out why he felt so sluggish at the start of this season. His doctors eventually gave him an explanation: The world's No. 1 tennis player had mononucleosis.

"Roger found out he had mono, and didn't know it. He found out a couple of weeks ago. That's probably what was wrong with him," Federer's agent, Tony Godsick, said today. "He doesn't want to make any excuses and take anything away from anybody. In hindsight, had (his doctors) known he had mono, they wouldn't have let him play."

Federer lost to eventual champion Novak Djokovic in the Australian Open semifinals in January, then didn't play again until this week, when he was upset by Andy Murray in the first round of the Dubai Tennis Championships. Federer had not lost in the first round of a tournament since 2004.

He was diagnosed with mononucleosis last month after seeing doctors in Switzerland and Dubai, Godsick said. Mononucleosis is an infection caused by a virus. Symptoms include fever, sore throat, headaches and feeling tired.

"This gave him a reason for why he wasn't able to move and why he wasn't able to recover," Godsick said. "It was annoying for him to not have reasons for why his body wasn't responding."

Federer was cleared to start practicing five days before the start of the Dubai tournament.

Godsick said Federer is "over it now," and will play Monday night against Pete Sampras in their exhibition match at Madison Square Garden.

"It's good to get an extra match under his belt," Godsick said. "He's looking forward to the Garden, 19,000 people, playing his idol. He's been somewhat down because he didn't know what was wrong with him."

Federer owns 12 Grand Slam singles titles, two away from Sampras' career record.

"I was feeling so great in December up until the moment I got sick, so this has been my problem the last couple weeks: really getting back on track," Federer told The New York Times for a story posted on its Web site Friday. "I haven't practiced and couldn't really work out the way I wanted to, because you have to be very careful with mono."