Russian third seed Maria Sharapova withdrew from the U.S. Open with a right leg injury, easing the path to a calendar-year Grand Slam for top-ranked Serena Williams.U.S. Open tournament director David Brewer announced Sharapova's exit, which allowed qualifying lucky loser Daria Kasatkina of Russia into the draw in Sharapova's place.Sharapova had been the highest-seeded rival in Williams' half of the U.S. Open draw, a status that now belongs to seventh-seeded Serbian Ana Ivanovic, a possible semi-final foe for Williams.Williams is trying to complete the first Grand Slam calendar-year title sweep since Steffi Graf in 1988 and match Graf's Open Era-record of 22 Slam singles crowns.