AMERICAN John Isner is out of the Australian Open.

The towering baseliner, ranked 13th in the world, withdrew this morning because of a knee injury.



"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."



He will be replaced in the main draw by a lucky loser from the last round of qualifying.



Isner was knocked out of the Sydney International by compatriot Ryan Harrison.





media_camera Sam Stosur practising today. Picture: Michael Klein























Meanwhile, Sam Stosur has began the final phase of an uncertain Australian Open preparation, returning to Melbourne Park to practise with Wimbledon winner Petra Kvitova.



US Open winner Stosur and Kvitova, both struggling for decent form, contested a practice match on Margaret Court Arena.



Stosur appeared to move without any problems despite recent ankle surgery and opening-round losses in Brisbane and Sydney and had the best of several rallies.

Australia's women's qualifying started disastrously with heavy opening-round losses for Sally Peers and Azra Hadzic.



Peers lost 6-0 6-0 in 46 minutes to Romanian Cristina Mitu before Hadzic fell to Portugal's Michelle Larcher De Brito 6-1 6-1.

Originally published as Isner out of Oz, Stosur's ankle OK