(CNN) -- U.S. Open champion Juan Martin del Potro has denied rumors that he is practising again following his recent wrist surgery, in comments that make it clear he is unlikely to be fit enough to defend his Flushing Meadows title in September.
The Argentine -- who has not been seen in action since January's Australian Open -- underwent surgery in May in an attempt to cure the long-standing problem to his right wrist.
Recent reports claimed that the 21-year-old had picked up a racquet again in preparation for his comeback but -- speaking on his Twitter page -- Del Potro has quickly denied he is on the comeback trail.
"I haven't picked up a racquet yet so I can't tell you an exact date for my return," del Potro wrote on his Twitter account. "When I know I'll tell you the news here. I'm improving, that's a good thing."
Speaking before his operation, Del Potro had told the official ATP Tour website: "I have tried all season to avoid surgery by attempting different treatments but, evidently, the injury was more serious than we thought.
"This is not a happy moment in my life, but I'm used to battling through adversity and I have the strength to come through this.
"The recovery time will be lengthy, but it depends on many factors that I can't assess because it's too soon."
Del Potro, who has slipped to seventh in the world from a career-high fourth since his absence, shocked Roger Federer's attempts to win a sixth successive U.S. Open title in a thrilling final last September.
He also became the first person ever to beat both Rafael Nadal (semifinals) and Federer in the same grand slam tournament.