What's the secret to get the better of Serena?

'There's no one as tough mentally as Serena.'

There is no secret to beating Serena Williams, says her US Open semi-final opponent Victoria Azarenka, the only chance to win is to fight hard, execute well and remember the 23-times Grand Slam champion is never out of it, no matter the score.

Azarenka, 31, set up the semi-final clash against Williams with a 6-1, 6-0 demolition of Belgium's 16th seed Elise Mertens on Wednesday.

While she has a 4-18 career record against Williams, the Belarusian was one of the few players who consistently challenged the American when in her prime from 2012-16.

"It's just a good execution and being able to fight for every ball," a smiling Azarenka said.

She made it clear to reporters there is no secret to beating Williams -- though added that even if there is she would not be sharing it.

"We had so many great battles when I won or when I lost. A lot of good matches," she added.

"I always know one thing: the match is never over. No matter what the score is, the match is never going to be over."

It will be Azarenka's first Grand Slam semi-final since 2013 when she went on to make her second successive final against Williams at Flushing Meadows.

Williams won both those finals, which are part of her 10-0 record against Azarenka in Grand Slams, but the Belarusian only had fond memories of playing the American.

"I like playing against the best," she said. "That's where you see yourself, your level, the biggest challenge.

"There's no one as tough mentally as Serena.

"I think we've built a lot of memories to remember. There's been a lot of great matches."