MELBOURNE - Serena Williams's Australian Open ended with her ankle swollen to the size of a balloon, back spasms and a quarterfinal loss to a young pretender on Wednesday, a two-week period that she described as the worst she had experienced at a Grand Slam.

"I'm almost relieved that it's over because there's only so much I felt I could do," the 15-times Grand Slam singles champion said after her 3-6 7-5 6-4 loss to American compatriot Sloane Stephens on Rod Laver Arena.

"Oh my gosh, it's been a little difficult. I've been thrown a lot of balls these two weeks.

"I've had a tough two weeks between the ankle, which is like this big every day," she added gesturing with her hands. "And my back, which started hurting. A lot of stuff."

Serena Williams of the US slides during her women's singles quarterfinal match against compatriot Sloane Stephens at the Australian Open tennis tournament in Melbourne, Jan 23, 2013. [Photo/Agencies]

Williams had entered the tournament as a hot favorite to claim a sixth Australian Open, having won the previous two Grand Slam titles, an Olympic gold medal and the season-ending WTA Championships on the way to a 36-1 record since Wimbledon.

An innocuous-looking slip, however, on Hisense Arena in the first round against Edina Gallovits-Hall set in motion two weeks of frustration.

The ankle became the subject of discussion after every match, despite her relative easy progression through the draw. She dropped just eight games into the quarterfinals.

The match against Stephens was billed as a showdown pitting the future of American tennis against the most accomplished player of the present generation, and the early signs indicated that Williams possessed too much power for her younger opponent.

Both players hammered away at each other on serve in the opener with the receiver winning a total of two points until the eighth game, when Williams broke Stephens to take a 5-3 lead then held to seal the set in 28 minutes.