RIO DE JANEIRO - Juan Martin del Potro slammed the brakes on his Olympic Games expectations Monday, believing he may not have the staying power to last the course. The Argentine star followed up his shock first round defeat of Novak Djokovic by downing Portugal's Joao Sousa 6-3, 1-6, 6-3 to move into the last-16. But del Potro is acutely aware that his frail wrists, which have required three surgeries, may torpedo his medal aspirations. "I am getting better and I am in good shape but I don't know if I am prepared to play six or more matches this week," said the 27-year-old former US Open champion. Del Potro has a heavy schedule in Rio. He is also playing men's doubles with Maximo Gonzalez. They face Spain's Rafael Nadal and Marc Lopez for a quarter-final slot later Monday. Del Potro, who missed most of the 2014 and 2015 seasons through injury, is right to be cautious. After stunning Stan Wawrinka at Wimbledon this year, he then suffered a huge letdown in the following round in a weary loss to Lucas Pouille of France. "I am going slowly, I am trying to get better every day and I think I am doing well," added del Potro, a bronze medallist at the London Games in 2012.