The world number 13 suffered a bone bruise during the Hopman Cup and has been unable to shake off its effects ahead of Monday's opening day in Melbourne.
Isner out of Australian Open after suffering knee injury
John Isner has been ruled out of the Australian Open after failing to recover from a knee injury.
The world number 13 has suffered bone bruising in his knee and will be replaced in the draw by a lucky loser from qualifying.
The big-serving 27-year-old picked up the injury in last week's Hopman Cup and had to withdraw from the international mixed team competition.
In a bid to prove his fitness Isner took part in the Sydney International, but lost to fellow American Ryan Harrison in straight sets.
"I am extremely disappointed to announce that I have to withdraw from the #AusOpen," Isner said on Twitter.
"I have been feeling some discomfort in my knee and have recently learnt that I have a bone bruise.
"Doctors have told me that continuing to play on the knee could result in a more serious injury."
Isner's withdrawal leaves the United States with Sam Querry as their only representative among the 32 seeds in the men's draw for the tournament, which starts on Monday.
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