MELBOURNE, Australia (AP) John Isner has withdrawn from the Australian Open because of a knee injury, leaving No. 22 Sam Querrey as the highest-ranked U.S. man at the season's first Grand Slam tournament.

Isner, who spent 18 weeks in the top 10 before finishing last year at No. 14, injured his right knee at the Hopman Cup last week and had to withdraw from the international mixed team competition. He lost to fellow American Ryan Harrison in straight sets at the Sydney International on Wednesday.

Australian Open organizers on Thursday said Isner had bone bruising of the knee. Isner says: ``Doctors have told me that continuing to play on the knee could result in a more serious injury.''