Williams pulls out of Hopman Cup clash

Serena Williams' bid for a seventh Australian Open crown is in doubt after her troublesome left knee flared again.

Williams was forced to retire from her Hopman Cup clash with Australian Jarmila Wolfe on Tuesday night after hobbling through a set-and-a-half.

The world No.1 was trailing 7-5 2-1 when she decided to call it quits after receiving a short medical assessment.

A day earlier, Williams didn't play at all during America's 2-1 tie loss to Ukraine because of inflammation in her left knee.

Last November, her coach Patrick Mouratoglou revealed the knee problem is similar to the one that has hampered Rafael Nadal in recent years.

Williams fell just short of achieving a calendar grand slam last year.