Defending champion Rafael Nadal\'s Wimbledon campaign runs into the imposing 1.98m (6ft 6ins) figure of dangerman Juan Martin del Potro Monday while the rejuvenated Williams sisters eye another final date. World number one Nadal, the 2008 and 2010 winner, has the toughest-looking clash of the fourth round when he tackles Argentine del Potro, the man who blew him off court in the US Open semi-finals in 2009 for the loss of just six games. But the Spaniard, fresh from his sixth French Open title, can take heart from his form so far at Wimbledon where he has yet to drop a set. Del Potro, by contrast, had to play on all six days of the first week as heavy rain played havoc with the scheduling. \"It\'s always a big challenge to play against him,\" said Nadal of his 24th-seeded opponent who missed most of 2010 with a wrist injury. \"He\'s a very, very tough opponent and one of the best players in the world.\" Del Potro insists all the pressure will be on Nadal in their eagerly-awaited fourth round clash. \"It was two years ago when I beat him in New York. I don\'t know if I\'m still far away from that level,\" said del Potro. \"To beat Rafa you have to play unbelievable tennis.\" Second seed Novak Djokovic was given a stern workout in his third round encounter by Cypriot Marcos Baghdatis on Saturday where he dropped his first set of the competition. Djokovic, beaten just once all year, faces French left-hander Michael Llodra. \"He will be tough. I need to work on my serve and return and hope it\'ll work out for me,\" said the Serb. Six-time champion Roger Federer faces Russia\'s Mikhail Youzhny who he has beaten 10 times out of 10. Federer, chasing Pete Sampras\'s record of seven Wimbledon titles, won the last of his 16 majors at the 2010 Australian Open. \"I don\'t go through days thinking like, My God, I haven\'t won a Grand Slam in so long,\" said the Swiss, who, like Nadal has yet to drop a set at this year\'s tournament. \"It turns around fast. You don\'t cry over stuff like that.\" Fourth seed Andy Murray tackles flamboyant French shot-maker Richard Gasquet who was two sets to love ahead of the Scotsman in their fourth round meeting here in 2008 before the Scot rallied to win. The other last 16 ties see America\'s final representative Mardy Fish meet last year\'s runner-up Tomas Berdych while Polish qualifier Lukasz Kubot faces Feliciano Lopez who put out three-time runner-up Andy Roddick. Spain\'s David Ferrer takes on Frenchman Jo-Wilfried Tsonga while Australian qualifier Bernard Tomic, the first 18-year-old to make the fourth round in 21 years, goes up against former semi-finalist Xavier Malisse of Belgium. In the women\'s last 16, Serena and Venus Williams, with nine titles between them, are still on course for a fifth all-sister final. Both have made light of their lengthy lay-offs to make the last 16 where Serena faces France\'s Marion Bartoli, the 2007 runner-up, while Venus takes on Tsvetana Pironkova who knocked her out in the quarter-finals last year. \"Don\'t bet against me,\" was Serena\'s response to her chances of a fifth title despite her 11-month lay-off. She looked impressive in defeating Russia\'s Maria Kirilenko 6-3, 6-2 on Saturday, where she fired 10 aces and 32 winners. \"It\'s the best I\'ve played since I came back.\" Top seed Caroline Wozniacki will aim to make the quarter-finals for the first time when she faces gritty Slovak Dominika Cibulkova. But most eyes will be on 2004 champion Maria Sharapova who faces China\'s Peng Shuai. \"I haven\'t been past the fourth round for a few years so I am happy to get where I am. I hope to go one step further and even more,\" said Sharapova. The other last 16 ties see big-hitting Sabine Lisicki, the conqueror of French Open winner Li Na, face Petra Cetkovska of the Czech Republic while Austria\'s Tamira Paszek takes on Russia\'s Ksenia Pervak. Nadia Petrova of Russia meets fourth seeded Belarusian Victoria Azarenka while 2010 semi-finalist Petra Kvitova faces Yanina Wickmayer.