World number one and defending champion Novak Djokovic was facing Wimbledon humiliation on Friday before rain saved him from his worst defeat at the tournament since 2008.

Three-time winner Djokovic, who is halfway to the first calendar Grand Slam in 47 years, dropped the first two sets of his third round clash against American 28th seed Sam Querrey 7-6 (8/6), 6-1.

But after just 73 minutes of action, rain sent the pair running for cover off Court One, giving 12-time major winner Djokovic welcome sanctuary before play was called off for the day.

By the end of Friday, which had seen just three hours of action on outside courts, only 15 of the scheduled 32 ties had been completed.

That led organisers to decide that for the first time since 2004, and only the third time in history, play will take place on the middle Sunday

Djokovic, who has an 8-1 record over Querrey, has made at least the quarter-finals on his last seven visits to Wimbledon since losing to Marat Safin in the second round eight years ago.

In contrast, seven-time champion Roger Federer had the advantage of playing under the Centre Court roof and became the first man in the fourth round thanks to a 6-4, 6-2, 6-2 win over Britain's Daniel Evans

Federer, beaten in the last two finals by Djokovic, has been aided by a generous draw that pitted him against two lowly Brits in world number 772 Marcus Willis and then world number 91 Evans.

"I didn't make the draw but it's been fun playing against Marcus and now Dan," said Federer who now has 305 Grand Slam singles match victories, which moves him within touching distance of Martina Navratilova's record of 306.

The All England Club, which has suffered three severely-hit rain days in the first week, said that playing on Sunday was needed to catch-up.

In a further effort to cut the backlog, all men's second round doubles matches were reduced to best of three sets rather than five.

There were still four second round singles to finish including two-time champion Petra Kvitova against Russia's Ekaterina Makarova.

"The move has been made in order to reduce the backlog of matches and to allow The Championships to finish as scheduled," said a club spokesman.

World number one and defending champion Serena Williams defeated US compatriot Christina McHale 6-7 (7/9), 6-2, 6-4 on Centre Court, only just avoiding what would have been her earliest-ever Wimbledon defeat.

The six-time Wimbledon champion will face Germany's Annika Beck for a place in the last 16.

"It was a really good match. She played great and she always plays great against me," Williams said after seeing off the world number 65.

"I know mentally no-one can break me."

Five-time champion Venus Williams reached the last-16, seeing off Russian teenager Daria Kasatkina 7-5, 4-6, 10-8 in a tie featuring a rain stoppage when she held match point.

Williams, 36, was 7-6, 40-30 up on Kasatkina's serve in the final set on Court One when play was halted.

Seventy-five minutes later, the players returned with Kasatkina saving the match point, the second of the day.

But Williams eventually triumphed at the third time of asking when the 19-year-old dumped a forehand into the net.

Williams, the eighth seed, goes on to face Spain's Carla Suarez Navarro for a place in the quarter-finals.

"I'm not sure I've ever played a 10-8 set. That was pretty intense. All you can think is 'hold serve'. Easier said than done," said the American.

Juan Martin del Potro admitted he felt "alive again" after the injury-plagued former US Open champion sent Swiss fourth seed Stan Wawrinka crashing out.

Playing in his first Grand Slam tournament since the 2014 Australian Open, his 3-6, 6-3, 7-6 (7/2), 6-3 second round victory over Wawrinka brought back memories of his stunning 2009 US Open win before a series of wrist injuries pushed him to the brink of retirement.

The world number 165 from Argentina, who was a semi-finalist in 2013 in what was his last appearance at Wimbledon, next faces French 32nd seed Lucas Pouille.

"It feels amazing, I feel alive again," said the 27-year-old who has undergone three wrist surgeries in the last two years.