MIAMI (AFP) Serena Williams and Juan Martin Del Potro have withdrawn from the upcoming ATP and WTA hardcourt tournament here with injuries, organizers said Friday. Argentinas Del Potro had hoped to be back but is still nursing a wrist injury that wrecked his Australian Open campaign. Last years US Open champion first felt the injury in Shanghai last October and it failed to clear up going into the new season. Williams, a five-time Miami titlist, pulled out with a knee injury which she continues to rehabilitate following her victory at the Australian Open. The tournament announced that Venus Williams will launch her bid for a fourth title on Thursday night. She won in Miami in 1998, 1999 and 2001.