Defeating Serena is the 'ultimate challenge' for Bartoli

The pick of the women's matches will see the 2007 runner-up taking on Williams, who has not lost at the All England Club since the final of 2008.

Marion Bartoli knows she needs to be at her best to defeat Serena Williams today.

Marion Bartoli has described facing the reigning Wimbledon champion Serena Williams in the fourth-round later today as the ultimate challenge.

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The pick of the women's matches will see the former finalist taking on a player who has not lost at the All England Club since the final of 2008.

Williams is only the seventh seed, promoted from 25th in the world rankings, after missing most of the last year through injury and illness, but she looked in ominously good form in a 6-3, 6-2 victory over Maria Kirilenko on Saturday.

Bartoli is enjoying her best year since losing to Serena's sister Venus in the final in 2007 but, after two tough three-setters and a heavy schedule that has included a run to the semi-finals of the French Open and winning the title in Eastbourne, she certainly has her work cut out.

The No 9 said: "With Serena or Venus, you have to step up, especially here at Wimbledon. She [Serena] is the ultimate competitor. You never count her out of a match.

"I saw her in Eastbourne and it was the first match she played in almost 12 months, and she was 6-1 down in the first set and somehow she found a way to win against [Tsvetana] Pironkova, who is a very good grass-court player.

"She always finds a solution, she always plays her best when it matters the most. And obviously the record she has here, and it is the same for Venus, shows what great champions they are.

"Of course it will be the ultimate challenge for me, but I think if I just have the same attitude and same spirit, no matter what the results will be, I can still be proud of myself."

The clash will open the day on Court One, while Centre Court features a rematch between Venus and Pironkova, a shock winner in their quarter-final encounter 12 months ago.

Caroline Wozniacki, the No 1 seed, is yet to drop a set and the Dane will be looking to reach the quarter-finals at the All England Club for the first time when she meets Dominika Cibulkova.

"We grew up playing against each other in the juniors. So we know each other pretty well. It's going to be a game where I need to keep my serve up. I need to get a lot of balls back and try to take the initiative and make her run," Wozniacki said.

Maria Sharapova, the fifth seed, has also impressed so far and she knows she will need a very solid performance to see off China's Peng Shuai on Court Two.