IF Anna Wintour ever tires of magazines, she shouldn’t count on an alternative career as a ball girl. During Sunday’s semifinal US Open match between Rafael Nadal and Juan Martin del Potro, a ball landed right in the Vogue editor’s row. “Everyone started screaming, ‘Anna, get the ball!’ — but she didn’t move,” laughs an eyewitness seated a few rows away. “No one else was in her row or the two rows in front of her, so the ball just sat there.” Finally, says our source, “She went to go get it and put it in her purse. Everyone started cheering.”