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Williams crashes out of Australian Open

Serena Williams suffered a first ever defeat to Ana Ivanovic © Getty Images

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Serena Williams is out of the Australian Open following a 4-6 6-3 6-3 fourth round defeat to Ana Ivanovic.

Five-time champion Williams revealed after the match that she had suffered from a back problem in the build-up, but maintained Ivanovic fully deserved to take credit for the win.

Ivanovic broke in the first set but saw her own serve broken twice in response from the world No. 1 as Williams took first blood.

However, as Williams struggled with her movement across court - and a time violation when serving to stay in the match - Ivanovic struck to complete victory in just less than two hours.

"I almost pulled out. I'm such a competitor, I probably should have," Williams said. "I don't want to blame anything. I feel like Ana deserves all the credit. I feel she played unbelievable today. I think she went for her shots."

"It's not anything that's life threatening or anything. I feel like it will get better. I just need a couple days off. Unfortunately I don't have that in a Grand Slam."

Ivanovic, the 14th seed in this year's event, was "thrilled" with her victory, which will now see her face Eugenie Bouchard following her 6-7(5) 6-2 6-0 win over Casey Dellacqua.

"It's amazing," said the 14th seed. "Honestly, I went on the court and I just wanted to play the best I could. I'm so thrilled.

"I was very consistent. It's not easy playing such a champion, but she's also just a human and I know she has lots of pressure too. I just went out there swinging and I think I did that really well."

Flavia Pennetta shocked ninth seed Angelique Kerber 6-1 4-6 7-5 to reach her first ever Australian Open quarter-final, where she will face Li Na.

Pennetta took the first set in 24 minutes but, following a short break from court, Kerber rallied to level the match. However, Pennetta broke serve three times in the final set to advance into the final four.

"I'm a little old so I'm starting to enjoy [tennis] more. When you are young you want to win so much," Pennetta said.

Na cruised through her match with Ekaterina Makarova, dropping just two games in the 6-0 6-2 victory.

"At least she's [Pennetta] one day older than me, so I'll play an older player," Li joked. "So nobody will talk about my age."

Meanwhile, there were no problems for defending men's champion Novak Djokovic or David Ferrer as they both reached the quarter-final with few worries.

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