American superstar Serena Williams is tantalisingly close to equalling Margaret Court's record haul of 24 Grand Slam titles but it's far from a certainty given her shaky displays so far at Wimbledon.

The 37-year-old seven-time champion had to draw on all her strength -- physical and mental -- to get past unseeded compatriot Alison Riske in the last eight and faces another grass court loving opponent in Barbora Strycova in Thursday's semi-finals.

The 33-year-old Czech veteran -- the oldest player to play in a woman's Wimbledon semi-final for the first time -- has played some wonderful tennis and says she will step onto Centre Court "without any fear".

The other semi-final pits two seeds against each other, seventh seed and former world number one Simona Halep against eighth-seeded Ukrainian Elina Svitolina.

It says a lot about the turbulence of women's tennis that Williams is making her 12th semi-final appearance while of the other three only Halep has gone this far before, and that was back in 2014.

Williams is the colossus that still bestrides women's tennis, but there are chinks in her armour as have been exposed both at Wimbledon and in her last two Grand Slam finals.

Outplayed by Angelique Kerber in last year's Wimbledon final and then a spectacular meltdown in the US Open defeat by Naomi Osaka gives Strycova genuine hope of an upset.

Not that the diminutive Czech will require any as her bubbly character oozes optimism and her form guide is as good as any of the semi-finalists having ousted four seeds on her way to the semi-final.

Williams's clay court campaign was affected by a knee injury but has accrued invaluable extra game time by playing the mixed doubles with another former world number one, Andy Murray.

"This is the first time since Australia (she reached the quarter-finals of the Australian Open) that I actually felt, like, good," said Williams.

"It's been a really, really long year for me already, and hard year, because I'm usually not typically injured.

"I don't know where I am. I do know I feel good."

However, she has still looked vulnerable and Strycova's speed round the court and array of shots will pay dividends if she reproduces the leaden-footed performance she put up against Riske.

Williams's coach Patrick Mouratoglou said Court's record is why his star came back after giving birth and multiple injury problems.

"That's why she came back to playing tennis after having a baby and so many medical complications," he said.

"The effort she's put in, I've never seen something like this.

"You have no idea how hard she worked to come back to that level, and she came back for that, so it will probably mean a lot if she makes it."

Strycova for her part described Williams as a great champion and amazing athlete but if that indicated she was intimidated think again.

"I don't have fear,"

"I don't have such power like Serena, but I have other weapons.

"I will try to use them as much as I can. I will enjoy.

"I have really at this point nothing to lose."

Her coach Lukas Dlouhy says Strycova has never given up hope of the big day finally arriving in what is her 53rd Grand Slam.

"She always tried hard, she was always working at 100 percent, she never quit or did anything wrong," said Dlouhy.

Halep should prevail in the other clash, not only because of her greater experience and having won a Slam in last year's French Open, but she says she has at last taken to grass after not being best suited to it when she lost to Eugenie Bouchard in the 2014 semi-final.

Halep showed this in impressing in her previous two matches dealing with the partisan crowd in beating Coco Gauff and then coming back from 4-1 down and break points in the first set against Zhang Shuai in her quarter-final.

"I'm a different person," said the 27-year-old Romanian.

"Everything changed. I have a lot of experience now. I'm more confident.

"I love grass. It's first time when I say that."