Wimbledon 2011: Williams sisters and Nadal lead line-up

2010 Wimbledon champions Serena Williams and Rafael Nadal
Williams and Nadal won last year's Wimbledon titles

A day of drama is in prospect at Wimbledon on Monday, with all 16 of the men's and women's fourth round ties taking place.

Following Andy Murray v Richard Gasquet on Centre Court is Venus Williams against Tsvetana Pironkova, then Rafael Nadal faces Juan Martin Del Potro.

Serena Williams opens on Court One against Marion Bartoli before Novak Djokovic takes on Michael Llodra.

Mikhail Youzhny against Roger Federer completes the show courts schedule.

Wimbledon officials have made it clear there are no more tickets available for the show courts on Monday, with some fans having queued since Friday afternoon.

Defending champion Nadal has not dropped a set in his three matches so far at this year's Wimbledon but he could face his toughest test yet against former US Open winner Del Potro.

The Argentine player was ranked fourth in the world in 2009 but is working his way back up the rankings after nearly a year out with a wrist problem.

It is Del Potro's first time past the second round at Wimbledon and Nadal knows he will not have things all his own way.

"I think he's a complete player," said the Spaniard.

"It is always is a big challenge to play against him. He's a very tough opponent and one of the best players of the world.

"His ranking is much better than what the ranking says today. His level is much better than what the ranking says today.

"If we talk about level, when he's healthy, he's a top-five player".

Del Potro is aware of the size of the task ahead of him.

"It's not easy to be top five but I'm improving my game every day," he said after his straight-sets third round win over 15th seed Gilles Simon.

"That's a good sign for the future. But, of course, I want to be at the top as soon as possible. But it's a long, long road."

Djokovic has lost just once in 45 matches this year but says he can improve his game after a not entirely convincing third round win over Marcos Baghdatis on Saturday.

"Llodra is a great server. He's playing fantastically at Wimbledon and I think grass courts are his favourite surface," said the Serb.

"It's going to be a tough one. I need to work on my serve and my return and hopefully it can work well for me."

Venus Williams is going in search of a sixth Wimbledon title, and will certainly not be short of motivation against Pironkova, who knocked her out in last year's quarter-finals.

Venus's sister Serena went on to win the title for a third time last year but the reigning champion says her older sibling looks in winning form once again.

"She seems way more confident and she's playing way more confident," said Serena, who has barely played since last year's final because of serious injury and health problems.

"That's obviously in turn inspiring me. She's working really hard and doing really well."

Even without the inspirational form of her sister, Serena is already a formidable opponent, and Bartoli says playing her will be the "ultimate challenge".

"She [Serena] is the ultimate competitor. You never count her out of a match," she said.

"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."