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Sydney, Australia (Sports Network) - American John Isner has pulled out of the Australian Open due to a right knee injury.

Isner's knee hampered him in his loss to Ryan Harrison at the Sydney International tennis event, a final Australian Open tune-up.

The big-serving 27-year-old was the top seed in Sydney. He said after Wednesday's 6-4 6-4 loss to Harrison that he was thinking about pulling out of the year's first grand slam due to the injury.

"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."

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