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Sabalenka, the reigning Madrid Open champion from Belarus, needed three sets to win her second-round tie against Magda Linette and the No.2 seed did not have an easy ride against American Montgomery.

However, she eventually got over the line, triumphing 6-1 6-7 (5-7) 6-4 to tee up a last-16 meeting with another American, Danielle Collins, who defeated Jaqueline Cristian 3-6 6-4 6-1.

Sabalenka has now won 14 of her 17 matches at the Madrid Open, having won the title in 2021 and 2023.

Since the inception of the tournament, only Serena Williams (15) has won more of her first 17 matches at the event.

Collins, meanwhile, is having the time of her life as she plays through her last year on the WTA Tour.

The American has now reeled off 15 straight wins, and took the titles at both the Miami Open and Charleston Open in recent weeks. This will be her first appearance in the last 16 at the Madrid Open.

"I've been doing so well the last couple of weeks, I think the girls know that when they come out and play me, they've got to go for it," Collins said.

"That certainly was taking place the last two matches, some big shots that I've had to counter, and be able to react quickly."