MELBOURNE, Australia (AP) -- John Isner has withdrawn from the Australian Open with bone bruising in his right knee, leaving No. 22 Sam Querrey as the highest-ranked U.S. man at the season's first Grand Slam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 knee at the Hopman Cup in Perth last week and had to withdraw from the international mixed team competition.
He tried playing at the Sydney International, where he was seeded No. 1, but lost to fellow American Ryan Harrison in straight sets in 71 minutes on Wednesday.
"I have been feeling some discomfort in my knee and have recently learned that I have a bone bruise," Isner said in a statement released by Australian Open organizers on Thursday. "Doctors have told me that continuing to play on the knee could result in a more serious injury."
The big-serving American led the tour in aces (1,005) and won two titles in 2012. He beat defending Australian Open champion Novak Djokovic to reach his first ATP Masters 1000 final at Indian Wells last March before losing to Roger Federer.
Isner has never won a major -- his best run was at the 2011 U.S. Open when he reached the quarterfinals -- but he does have some notable claims to fame.
In 2010, he played the longest top-level match in tennis history, a marathon win over Nicolas Mahut at Wimbledon that spanned three days and lasted 11 hours, 5 minutes and went to 70-68 in the fifth set.
Last year, Isner tied a record for the latest finish at the U.S. Open before losing in the third round to Philipp Kohlschreiber of Germany 6-4, 3-6, 4-6, 6-3, 6-4 at 2:26 a.m.