Serena Williams thought she was heading to defeat to Heather Watson at Wimbledon

Serena Williams admits Heather Watson had her contemplating a shock Wimbledon exit before the American came through a three-set thriller on Centre Court.

Watson was up two breaks and led 3-0 in the third set, and the British No 1 later served for the match at 5-4, but Williams roared back to win 6-2 4-6 7-5.

Williams will now play her sister Venus in the last 16 as she looks to clinch her sixth Wimbledon title and 21st Grand Slam overall.

"I honestly didn't think I was going to win," Williams said. "How I pulled through, I really don't know.

"I just was like, 'listen, if I'm going to go lose, I'm going to lose trying to do the right things."'

Williams continued: "I'm actually more negative than one would think.

"I'm like, 'I guess I'm going to do this tomorrow'. I'm not really thinking about the match.

"Next thing, I'm playing a point, maybe it takes my mind off things.

"It's not that I thought I was going to lose. I just felt like she was playing really well. I wasn't able to keep up.

"Sometimes you just don't have your day. I thought maybe today just wasn't my day."

I honestly didn't think I was going to win. How I pulled through, I really don't know.

It means the Williams sisters will meet for a sixth time at Wimbledon, with four of their previous match-ups coming in the final at the All England Club, and Serena emerging victorious on three occasions.

In 25 matches against each other, Serena has won 14 times and Venus 11.

Venus may have the benefit of fresher legs, however, after the seven-time major champion eased to a 6-3 6-2 win over Serbian Aleksandra Krunic.