Tight back

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.

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

"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 quarterfinal 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 favor 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."