...Nikolay Davydenko of Russia serves the ball during the men's singles final match against Juan Martin Del Potro of Argentina during the Barclays ATP World Tour Finals at the O2 Arena on November...

...Federer of Switzerland returns the ball during the men's singles round robin match against Juan Martin Del Potro of Argentina during the Barclays ATP World Tour Finals at the O2 Arena on November...

LONDON, ENGLAND - NOVEMBER 26: Juan Martin Del Potro of Argentina celebrates going through to the semi-final after he won his match during...

Juan Martin Del Potro of Argentina plays the ball through his legs against Roger Federer of Switzerland in a...

Juan Martin Del Potro of Argentina gestures as he plays against Fernando Verdasco of Spain in a Singles match...

...ended his career with style with a 6-4, 5-7, 6-4 defeat by Juan Martin del Potro in the second round of the Paris Masters. The 29-year-old Safin, a two...

In his hometown of Tandil, Juan Martin del Potro has already arrived

Juan Martin del Potro upset No. 1 Roger Federer to win the US Open

Juan Martin del Potro, left, of Argentina, and Roger Federer, of Switzerland, argue with the chair umpire over a...

Juan Martin del Potro, of Argentina, waits for trophy presentations after winning the men's finals championship over Roger...

Juan Martin del Potro in action during his 2009 US Open Mens Championship match against Roger Federer in Arthur...

September 14, 2009: Argentina's Juan Martin Del Potro holding the Champions trophy after defeating Switzerland's Roger Federer in the Men's Singles...

NEW YORK - SEPTEMBER 14: Juan Martin Del Potro of Argentina celebrates winning the fourth set tiebreak during the Men's Singles final against...

Caption: NEW YORK - SEPTEMBER 14: Juan Martin Del Potro of Argentina returns a shot against Roger Federer of Switzerland during the Men�s...

NEW YORK - SEPTEMBER 14: Roger Federer of Switzerland serves against Juan Martin Del Potro of Argentina during the Men�s Singles final on day fifteen of the 2009...