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.
Isner 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 have been feeling some discomfort in my knee and have recently learned that I have a bone bruise," Isner said.
"Doctors have told me that continuing to play on the knee could result in a more serious injury."
Isner was the highest ranked American in the competition, leaving 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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