Ana Ivanovic dumped world No 1 Serena Williams out of the Australian Open with a fourth round 4-6 6-3 6-3 victory on Sunday.
The 14th seed had failed to win a single set in their previous four meetings but broke four times before serving out the match to love.
Despite serving 13 aces to Ivanovic's one, Williams was well below par throughout, particularly vulnerable on her second serve, winning just 41 per cent of points on it.
The five-time champion confirmed after the defeat that she has been struggling with a back problem and almost pulled out of the tournament.
It looked like another routine victory was on the cards for the French Open and US Open champion as she came through the opener, responding to an early break with two of her own.
However, the Serb earned eight break points during the second set and took two while being untroubled on her own serve.
She then broke in the second game of the decider and, after failing to take her first match point at 30-40 in the penultimate game, served out the match to love.
"I think all the struggles were just for moments like this," said the popular 26-year-old.
"I actually believed. I had some confidence coming into today's match. I really did certain things extremely well and I kept her under pressure I felt throughout the whole match.
"I just think I stayed in the moment physically. I didn't think much about the occasion and who I was playing, because it can get overwhelming."
Since winning the French Open in 2008 Ivanovic has only once reached the quarter-final stages of a grand slam, at the 2012 US Open.
"I really feel that the team that I have now is really behind me and they believe in me," she added. "I went through the tough times when maybe I doubted, but they don't. That means a lot to me."
Williams told reporters how serious her back injury was - but was magnanimous enough to admit Ivanovic won on merit.
"I almost pulled out. I'm such a competitor. I mean, I probably should have," she said.
"I have just been on the strongest meds I can take, that are legal, which my body didn't handle well because I haven't been taking anything.
"So I was like really kind of out of it for a few days. But that had nothing to do with it. I think Ana just played a really good match. She did what it takes to win."
Ivanovic will next play promising Canadian Eugenie Bouchard for a place in the semi-finals after she overcame Australian wildcard Casey Dellacqua 6-7 6-2 6-0.