Halep maintained her pursuit of a third title, and the number one ranking, with a 6-2, 6-7, 6-0 win over Switzerland's Belinda Bencic at the Premier Mandatory tournament at La Caja Mágica.

The Romanian, who claimed the Madrid Open title in 2016 and 2017, will replace Naomi Osaka as world number one if she lifts the trophy on Sunday (May 12).

Standing in her way will be Bertens, who beat 2017 US Open champion Sloane Stephens 6-2, 7-5.

In the Association of Tennis Professionals (ATP) World Tour Masters 1000 event, fourth seed and 20-time Grand Slam winner Roger Federer was beaten by Austrian Dominic Thiem.

Thiem saved match points before he clinched a 3-6, 7-6, 6-4 triumph over the Swiss star to reach the semi-finals.

It marked the fifth seed's second consecutive win against Federer, who he beat in the final at Indian Wells in March.

Thiem, considered a clay court specialist, will face Novak Djokovic in the last four after the Serbian's opponent, Croatia's Marin Cilic, withdrew due to food poisoning.

"The test today was huge and tomorrow is a different game, of course,” said Thiem. 

“But I was playing Novak last year, and two years ago, and he was not at his best, I guess. 

"Now he is again. 

"He's won the last three Grand Slams and he is at the top of the ATP ranking again. 

"So the challenge couldn't be bigger. 

"I'll try to keep up my good momentum which I have now and give my best again."