John Isner withdraws from Australian Open

American world number 14 John Isner has pulled out of the upcoming Australian Open with a knee injury.

The 27-year-old hinted on Wednesday that he could be forced to miss the tournament following his defeat to Ryan Harrison at the Sydney International.

"I am extremely disappointed to announce that I have to withdraw from the Australian Open," said Isner.

"I have been feeling some discomfort in my knee and have recently learned that I have a bone bruise."

The first Grand Slam of the season starts at Melbourne Park on Monday.