Tennis fans reckon they know the exact moment Serena Williams turned the tide in her tense win over Alison Riske at Wimbledon on Tuesday.

Williams scrapped her way into a 12th Wimbledon semi-final with a rollercoaster 6-4 4-6 6-3 victory over her giant-killing compatriot.

After dropping the second set and looking scratchy in the third, fans noticed a change in Serena at 3-3 in the decider.

Her hair.

After starting the match with it in a pony tail, Serena came back on court with it in a tight bun at 3-3.

She then won three-straight games to take the match, and fans didn’t think it was a coincidence.

Asked about the change of hairstyle in her post-match press conference, Serena said it was simply annoying her.

"Today it was just in my way--and the wind: I was missing a shot because it's in my face,” she said.

“I was like, ‘this is not happening’. I just needed to get it out of the way, put the business bun up and just get to business."

History beckons for Serena

Turning 38 in September and already the oldest grand slam champion in the open era after winning the 2017 Australian Open while pregnant, Williams is favourite to land a first major as a mother - and eighth overall on London's famous grass courts.

And, incredibly, the former world number one is still challenging for grand slam spoils - picking and choosing her tournaments while juggling family life with less and less tennis.

Without a competitive outing between her 2018 US Open final meltdown against Naomi Osaka and the Australian Open in January, Williams arrived last week having not played a match since the French Open more than a month ago - and only a dozen all season.

Yet she still managed to end Riske's brilliant grass court run with 18 aces and six service breaks in another nerveless Centre Court display.

"I believe in myself. I believe if I'm feeling well, I can be a big competitor in a sport that I love and I've done so well at," Williams said.

"So just the key word is getting fit and getting back into match play injury-free."

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Desperate to match Margaret Court's record 24 singles majors, Williams will play unseeded Czech Barbora Strycova on Thursday for a place in Saturday's title decider.

Strycova crushed British hopes with a 7-6 (7-5) 6-1 win over Johanna Konta in Tuesday's last quarter-final.

Williams' fellow former world No.1 Simona Halep took out Zhang Shuai 7-6 (7-4) 6-1 to advance to the last four for the first time since 2014.

The Romanian seventh seed will face eighth seed Elina Svitolina, a 7-5 6-4 victor over over Czech surprise packet Karolina Muchova.

WOMEN'S SEMI-FINAL MATCH-UPS:

11-Serena Williams (USA) v Barbora Strycova (CZE)

7-Simona Halep (ROM) v 8-Elina Svitolina (UKR)

with AAP