Swiss tennis player, Roger Federer (R) and Argentine tennis player Juan Martin Del Potro attend a press conference in the city of Tigre, near Buenos Aires, capital of Argentina, on Dec. 11, 2012, ahead of two exhibition matches which will be held on December 12 and 13. (Xinhua/Martin Zabala)

Swiss tennis player, Roger Federer attends a press conference in the city of Tigre, near Buenos Aires, capital of Argentina, on Dec. 11, 2012, ahead of two exhibition matches which will be held on December 12 and 13. (Xinhua/Martin Zabala)

Swiss tennis player, Roger Federer (R) and Argentine tennis player Juan Martin Del Potro attend a press conference in the city of Tigre, near Buenos Aires, capital of Argentina, on Dec. 11, 2012, ahead of two exhibition matches which will be held on December 12 and 13. (Xinhua/Martin Zabala)

Swiss tennis player, Roger Federer (R) and Argentine tennis player Juan Martin Del Potro attend a press conference in the city of Tigre, near Buenos Aires, capital of Argentina, on Dec. 11, 2012, ahead of two exhibition matches which will be held on December 12 and 13. (Xinhua/Martin Zabala)

Argentinian former tennis players Guillermo Vilas (L) and Jose Luis Clerc hug during their press conference in the city of Tigre, near Buenos Aires, capital of Argentina, on Dec. 11, 2012, ahead of two exhibition matches which will be held on December 12 and 13. (Xinhua/Martin Zabala)