American world No 13 John Isner has withdrawn from next week's Australian Open with a knee injury, tournament organizers said on Thursday.

The big-serving Isner, who was seeded 12 at the first Grand Slam of the season, injured his right knee during the Hopman Cup in Perth last week and was forced to withdraw from the event.

He had further problems during his straight-sets loss to fellow American Ryan Harrison in his first match at the Sydney International on Wednesday.