FEDERER HAS MONO

Roger Federer's most intriguing season continues on Monday with a singles match in New York that won't count in the rankings but has still managed to sell out 19,000 seats in Madison Square Garden.

Federer's latest exhibition duel with the now-retired Pete Sampras is just that: an exhibition. But it comes at a particularly sensitive stage of Federer's brilliant career; one where he has looked surprisingly vulnerable to the slings, arrows and energy of the younger generation.

He has played just two tournaments in 2008 and lost twice to 20-year-olds, with Novak Djokovic manhandling him in straight sets in the semifinals of the Australian Open, and Andy Murray upsetting him in a tighter match this week in the first round in Dubai after Federer had taken a five-week break from competition.

In both defeats, Federer looked less fleet and fluid than usual and also less than convincing with his signature forehand.

Crisis? Beginning of the end of his long reign? Perhaps, but Federer, as it turns out, has not been practicing full disclosure until now.

At age 26, he has not only been struggling with young, gifted and hungry opposition, but also his health. Last month, after falling ill for the third time in six weeks, he underwent extensive tests in his native country of Switzerland and his part-time residence of Dubai. According to Federer, the conclusion was that he had contracted mononucleosis.

Federer already had been diagnosed with food poisoning prior to the Australian Open, which severely disrupted his preparation for that tournament, eventually won by Djokovic. But Federer, who complained of feeling sluggish during the Open, said it now appears that the mononucleosis was the more serious issue.

Mononucleosis is an infection caused by the Epstein-Barr virus. It can produce flu-like symptoms and extreme, lingering fatigue. Physicians often discourage those with mononucleosis from taking part in intense physical activity because of the risk of rupturing the spleen, which can become enlarged because of the infection.

He was well aware that mononucleosis forced Mario Ancic, a former top 10 player from Croatia, to miss six months of the 2007 season, including Wimbledon.

Federer said he was unable to practice for about 10 days in February and received medical clearance to begin training normally five days before the tournament in Dubai began on Monday.

"They weren't sure I was over it, but now I'm creating antibodies and this really shows you are over it," he said.

Even in perfect health, this season looked like the most challenging of Federer's career with the emergence of Djokovic and with an overstuffed calendar that includes the Olympic tournament in Beijing. He is still on a quest to win his first French Open, the only Grand Slam singles title he lacks, and also will be attempting to break his tie in the record books with Bjorn Borg by winning a sixth straight Wimbledon.

Now, 2008 looks even more challenging, and it should be fine theater to see how a champion accustomed to winning big titles without much adversity will react.

Until now, Federer has been largely spared any major health concerns and has played in 33 consecutive Grand Slam tournaments, winning 12 of them: two short of Sampras's all-time record. He won three of the four major titles in 2006 and did the same in 2007, but despite his stuttering start in 2008, he said it is too early to claim that his era of dominance is ending.