Tennis: Serena crashes as Li fires up

MELBOURNE - Serena Williams was a shock casualty in the Australian Open fourth round Sunday as Ana Ivanovic scored the tournament's biggest upset, but it was plain sailing for Li Na.

Injured five-time champion Williams was bundled out 4-6, 6-3, 6-3, ending her bid to match Chris Evert and Martina Navratilova with 18 Grand Slam titles.

The American revealed afterwards that she had a back injury and had almost pulled out of the tournament before her third-round clash with Daniela Hantochova.

But she refused to blame the problem for her defeat.

"Maybe I wasn't the best physically, but that had nothing to do with it," she said. "I think Ana just played a really good match. She did what it takes to win."

Williams, whose movement was clearly restricted, pulled out of the doubles on Friday, although the withdrawal was blamed on an injury to sister Venus.

It was a major scalp for Serbia's Ivanovic, 26, and set up a quarter-final against fast-improving Canadian teenager Eugenie Bouchard who ended the hopes of Australia's Casey Dellacqua in three sets.

Since winning at Roland Garros in 2008, Ivanovic, ranked 14, has only made a Grand Slam quarter-final once, at the 2012 US Open.

Despite her good form, which has included winning the Auckland International ahead of Melbourne Park, she is not getting ahead of herself.

"The only goal I set for myself is to keep this level and to give my best every match possible," she said. "There are tough opponents, dangerous opponents each round.

"I'm just so thrilled to reach quarter-final here, first time since 2008, and it means so much to me to do it in this manner, too."

In contrast to her battle to overcome Williams, Li's match with with Russia's Ekaterina Makarova was mundane as the Chinese fourth seed made amends for her lacklustre showing in the third round.

Li crushed the 22nd seed 6-2, 6-0 in less than an hour to reach a quarter-final against Italy's Flavia Pennetta, who battled past ninth-seeded German Angelique Kerber 6-1, 4-6, 7-5.

Two-time Melbourne finalist and 2011 French Open winner Li was rarely extended as she moved within two matches from her third Melbourne decider.

Li made amends after for failing to impress her coach Carlos Rodriguez during the third round, when she had to fight through a tiebreak and save a match point.

"I think yesterday I did a lot of practice because I really made Carlos sad two days ago during the match," she said, explaining her improved performance against Makarova.

"So, yeah, I try a lot yesterday and also today in the match. But at least today I think made him a little bit happier. Feel much better."

Three more victories will end her jinx at the season's opening Grand Slam.

The 31-year-old led the 2011 final before losing to Kim Clijsters, and last year she was ahead against Victoria Azarenka but dramatically twice rolled her ankle and briefly blacked out.

In beating Kerber, Pennetta reached the last eight at Melbourne Park for the first time since making her tournament debut in 2003.

It continues her strong form at Grand Slams, having made the semi-finals at the US Open last year.

"I missed last year so it is good to be back and I'm shaking," said the Italian, who was out last year with a right wrist injury that needed surgery.