(CNN) -- Roger Federer will be able to bounce back again following his shock U.S. Open final defeat, tennis legend Boris Becker has told CNN.
Roger Federer suffered his first defeat at Flushing Meadows since his fourth-round exit in 2003.
The five-time winner went into Monday night's final at the top of his game, having re-established himself as the top player in the world this year after losing his No. 1 crown to Rafael Nadal and suffering from illness.
The Swiss star was widely expected to defeat Juan Martin Del Potro, the 20-year-old sixth seed from Argentina who reached his first Grand Slam final after crushing an injury-hampered Nadal in the last four.
"It was a very big upset in world tennis," said Becker, who won the U.S. Open in 1989 as one of his six Grand Slam titles.
See Becker's interview with CNN.
"Everybody had their money on Federer obviously, and even last night how the match went after the third set when Federer, two sets to one up, you expected the 15-time Grand Slam champion to go through.
"But obviously Del Potro had a different plan and he came back in a fantastic tiebreak in the fourth set and Federer actually looked a bit tired in the last stages of the fifth set."
Federer completed his set of Grand Slam titles in June after finally winning the French Open on clay -- his least favorite surface -- and regained his Wimbledon crown in an epic final with Andy Roddick after Nadal missed the grass tournament due to his knee problems.
The 28-year-old went into the New York hardcourt event on Cloud Nine after becoming a father to twin baby girls, having married longtime partner Mirka Vavrinec in April, but saw Del Potro emerge as a real threat to his dominance as Nadal and Andy Murray fell away.
"He's in a difficult phase in a high-comfort level because there is nothing else to prove anymore, to himself or the fans," Becker said.
"So I think he is going to go back and enjoy his new babies and new wife by then he is going to regroup and restructure for next year's Grand Slams."
Federer has won the Laureus World Sportsman Of The Year title for the last four years, but Becker believes he may miss out in 2009 to Jamaican sprinting star Usain Bolt -- who followed up his triple golds from the 2008 Beijing Olympics with more records in winning three titles at the World Athletics Championships.
"Well yesterday evening at two sets to one for Federer it would have been a sure bet," Becker said. "But him not winning now puts everything in a new race and Usain Bolt winning other gold medals at the world championships obviously is a strong frontrunner."
The U.S. Open also saw a surprise in the women's singles, where Kim Clijsters capped her amazing comeback to claim back the title she last won in 2005.
Becker said the Belgian's heroics, coming in only her third tournament back on court after retiring to have a baby, surprised him as much as everyone else.
"I was positively surprised for her to come back and just play some tennis again, but I never expected in my wildest dreams for her to actually come back and beat the Williams sisters and actually win the whole Grand Slam, the whole U.S. Open," the 41-year-old German said.
"It is one of the greatest stories I've heard in sport in along time."
Clijsters' 2007 victory was, up until this weekend, her only Grand Slam title.
"Probably in her past she was really sort of a choker sometimes in the semifinals and finals, that's why she only won one Slam," Becker said.
"I think her daughter made all the difference in her life, her perspective was in order. Physically she was always strong, so at 26 years of age you don't really lose a step of speed or endurance. I think she is overall more relaxed and therefore she can focus more on her tennis."
Williams has apologized after her outburst at a line umpire who called her for a foot-fault while serving at championship-point down, which handed Clijsters the title.
See pictures of Serena's meltdown.
The American, who has already been fined, should not face any further punishment, according to Becker.
"That would be a mistake. I think Serena immediately after the match regretted what she said, she was very apologetic and she knows that the biggest penalty she had was losing that match.
"In a perfect scenario you shouldn't swear at the umpire or the linesman, but I was also one of the guys who did that -- thank God in my days we didn't have that many mics on the court, so you couldn't really hear everything.
"It is really unfortunate for her to do it at match-point down. It is really something for such an experienced player. It shouldn't have happened, but she is human. I think she apologized after, but that's the reason she lost, I think, and that's the biggest penalty she can ever get."