John Isner has withdrawn from the Australian Open due to a knee injury, leaving No. 22 Sam Querry as the highest-ranked U.S. man at the season's first Grand Slam tournament.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.
Isner announced Thursday he had to pull out of the Australian Open, saying: ''Doctors have told me that continuing to play on the knee could result in a more serious injury.''