MELBOURNE - Luckless Grand Slam winner Juan Martin del Potro Sunday pulled out of Australian Open after a recurrence of his troublesome wrist injury, but remains confident of resurrecting his career. The Argentine spent almost a year on the sidelines before making his return as a wildcard at the Sydney International last week, where he made the quarter-finals. But he said he was not ready for gruelling five setters. “I didn’t feel prepared for playing in this big event. My first Grand Slam after one year, five-set matches, is very tough,” said Del Potro , who memorably won the US Open in 2009 at the age of 20 when he defeated Roger Federer. “It could be dangerous for my wrist. I think it was a tough decision, but could be a smart decision if I want to stay healthy.”