Ana Ivanovic of Serbia serves to Serena Williams of the US during their women's singles match at the Australian Open 2014 tennis tournament in Melbourne January 19, 2014. [Photo/Agencies]

MELBOURNE, Australia - The powerful serve that carried Serena Williams to 25 consecutive wins suddenly became the target of Ana Ivanovic's attack in a major fourth-round upset at the Australian Open.

Williams had a back complaint she'd tried to keep secret before Sunday but Ivanovic noticed almost immediately that her serve wasn't as fast as she'd expected, and she started taking big swipes at it.

The No. 14-seeded Ivanovic ended Williams' bid for a sixth Australian and 18th Grand Slam title with a 4-6, 6-3, 6-3 victory that took the hottest of favorites out of contention at Melbourne Park and opened up one side of the women's draw.

"It's not easy playing such a champion ... but she is also just a human," said Ivanovic, who beat Venus Williams in the final at Auckland, New Zealand earlier this month. ``I just went out there swinging.''

Williams hadn't lost a match since August, and went 78-4 in 2013. She came into the fourth round with those 25 straight wins, the second-longest streak of her career, and it was her 70th match at Melbourne Park, a record in the Open era. Williams had just set the mark for most match wins (61) ever at the Australian Open with her third-round victory.

The No. 1-ranked Williams didn't move her feet well, and was lunging for balls. When she tried to step up the intensity in the third set, her grunts becoming louder and more frequent until she toned it down after spraying a wild backhand wide in the next-to-last game, Ivanovic matched her stroke for stroke.

"It wasn't the best," Williams told a news conference, after being told that her coach had leaked news of a back problem.

"Again, I don't want to blame anything. I feel like Ana deserves all the credit," Williams added. "I feel she played unbelievable today. I think she went for her shots. It's not like I gave her the match."

Ivanovic advanced to the quarterfinals here for the first time since her run to the 2008 final and will next play the winner of Sunday's later match between Australian wild-card entry Casey Dellacqua and No. 30-seeded Eugenie Bouchard.