* Ankle, back injuries sour Serena's Australian Open
* No chance of retiring (Writes through with details, quotes)
By Greg Stutchbury
MELBOURNE, Jan 23 (Reuters) - Serena Williams's Australian Open ended with her ankle swollen to the size of a balloon, back spasms and a quarter-final 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."
Williams had entered the tournament as a hot favourite 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 quarter-finals.
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.
The turning point came in the second set when the 31-year-old raced forward to a wickedly backspun drop shot and a "tight back" that had been effecting her for a few days immediately flared.
"It just locked up on me," she said. "I even screamed on the court. I was like, 'Ahh!' It was a little painful.
"I couldn't really rotate after that, which I guess is normal."
Williams is renowned for her toughness on court but such was the mask of despair and pain on her face, she looked as if she could retire from the match. The world number three, however, refused to countenance any such suggestion.
"Are you kidding me?" she asked. "I mean, I thought about it like for a nanosecond. It's a quarter-final of a grand slam.
"I have to (be) taken off in a wheeler before I retire."
Williams did take a medical time out and when she returned she was finding it hard to serve or rotate to her backhand, but she still managed to put pressure on the 19-year-old, breaking her once when she was serving for the set.
The teenager, however, hit back at the next opportunity and the deciding set turned into a match fraught with tension after Williams exploded in a fit of rage at the end of the third game, smashing her racquet.
The momentary outburst of destruction appeared to work in her favour as she looked more comfortable in her movement, and broke to make it 4-3 in the decider but was unable to prevent her gutsy opponent from breaking back immediately.
Stephens then held and broke again to seal the shock victory when a Williams backhand splattered into the net.
"I think my opponent played well and was able to do a really good job," Williams said in acknowledging the back injury was not an excuse for the loss.
"I don't think my level was high. I don't know how many unforced errors I ended up hitting, but for sure, more in one set than I probably did in the whole tournament.
"So I think her level was high. But as for my end... you can tell... this is definitely not my best match in months." (Editing by Patrick Johnston)