Serbian Ana Ivanovic has pulled off the upset of the Australian Open so far with defeat of top seed and overwhelming tournament favourite Serena Williams.
Ivanovic, the 14th seed, won their fourth-round match 4-6 6-3 6-3 at Melbourne Park on Sunday.
The 2008 French Open champion hadn't won a set against Williams in their four previous meetings but she troubled a lethargic world No.1 from the opening game.
"It's not easy playing such a champion," Ivanovic said.
"But she is just human."
Afterwards, Williams revealed she almost pulled out of the Open with a back injury.
She was reluctant to discredit her opponent - a three-time grand slam finalist and former French Open champion and world No.1 - but admitted the injury contributed to her loss.
The American initially deflected questions about the injury but came clean after being told her coach had already let the cat out of the bag.
"I guess the secret is out, but I obviously wasn't hitting the way I normally would hit and wasn't moving the way I normally would move and making a lot of errors that I normally would not make and I haven't made in a couple of years," Williams said.
"But it's OK. I feel like I know for a fact I can play so much better than what I did today, so with that, knowing that, I'm not disappointed or anything.I just know that I can play 10 times better than what I did today.
"I made a tremendous amount of errors, shots I missed I normally don't miss - I haven't missed since the '80s."
Williams, who has won 17 majors, was hoping to catch tennis legends Martina Navratilova and Chris Evert on the all-time grand slam leaderboard with a sixth Open title at Melbourne Park.
"I almost pulled out. I'm such a competitor. I mean, I probably should have," Williams said.
"But I don't want to blame anything. I feel like Ana deserves all the credit.
"Maybe I wasn't the best physically, but that had nothing to do with it.
"I think Ana just played a really good match. She did what it takes to win.
"I feel she played unbelievable today. She went for her shots. It's not like I gave her the match.
"I tried to fight the best I could today. But, yeah, I almost didn't play.
"But, hey, I did, and at least I feel good that I tried the best that I could."
Williams is turning 33 in September but after winning 78 of 83 matches in a dominant 2013 season, she is confident of adding to her record Australian Open collection.
"I feel like I'll win it again," she said.
"Obviously not this year, but maybe next year."
Ivanovic will face either Australian Casey Dellacqua or Canadian 30th seed Eugenie Bouchard in the quarter-finals.
Ivanovic joins two-time Australian Open finalist Li Na in the quarter-finals after the Chinese star scored a dominant fourth-round victory over Russia's Ekaterina Makarova.
The fourth-seeded Li took less than an hour to dispose of Makarova 6-2 6-0, setting up a quarter-final against Italy's Flavia Pennetta.
Li has led in both her finals in Melbourne - against Kim Clijsters in 2011 and Victoria Azarenka last year when severely hampered by an ankle injury.
Pennetta, the 28th seed, endured more serious opposition from German ninth seed Angelique Kerber before registering one of her best grand slam wins 6-1 4-6 7-5.
Despite being closer to the end of her career than the beginning, Pennetta's run is her best in 10 appearances at the Australian Open and follows her best-ever grand-slam performance when a semi-finalist at last year's US Open.
Li broke Makarova in the third game of the opening set, establishing a superiority that was barely threatened.
A second break gave Li a 5-2 lead and she duly served out for a 1-0 lead.
The only lapse in Li's performance came in the fourth game of the second set when she offered her opponent two break points which she failed to convert.