London: Serena and Venus Williams on Friday survived marathon duels as the rain, which has plagued the tournament, forced organisers to consider playing on the middle Sunday for the first time since 2004.
World number one and defending champion Serena 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. I knew, down a break in the third, that I was going to have to put my mind in it and that's what I did."
Serena was far from her best, even picking up a code violation for smashing her racquet into the turf at one stage.
Five-time champion Venus 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.
"When we came off on match point, it was like something out of a movie."
On Friday, only one minute of play was possible on outside courts in four hours and there was a further 75-minute stoppage later.
The tournament already had a backlog of 16 second round ties and with more rain expected Saturday, organisers admitted that play may take place on Sunday for the first time since 2004 and only the third time overall.
