LONDON: World number one Serena Williams survived an almighty scare at Wimbledon on Friday, battling back to beat Britain's Heather Watson in three sets and stay on course for a 21st grand slam title.

Williams, who has lifted this year's Australian and French Open titles, was forced to dig deep in front of a raucous home crowd after an error-strewn showing against the superb Watson, recovering from 3-0 down in the final set to win 6-2 4-6 7-5.

The 33-year-old, who said it was one of her toughest matches, will face her sister Venus in the last 16 -- their 26th career meeting -- as she continues her bid for a sixth Wimbledon title.

The signs looked ominous for Watson early on after being swept aside in the first set but the 23-year-old bounced back in the second, forcing Williams into a string of wayward shots as she matched up to her more powerful opponent.

Watson reeled off six games in a row with three straight service breaks to level the match and race into a 3-0 lead in the decider.

Even after a Williams rally she was twice just two points away from an unlikely win, only for the American to show the grit and guile behind many of her successes to prevail.