Simona Halep set up a quarter-final clash with Ons Jabeur after battling past a spirited Coco Gauff.
Gauff was the higher ranked player but by no means the favourite against her illustrious opponent, and Halep's experience showed as she tempered a fierce start from Gauff.
The youngster flew out of the blocks and held to love twice in her opening four matches, with Halep just about going toe-to-toe on serve to keep the score level.
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But Halep was unwavering while an aggressive and spirited Gauff lost her rhythm in the ninth game, double faulting to hand Halep a break which she consolidated to take the set.
Gauff returned in the second set and broke her opponent immediately to threaten a comeback.
But Halep was steely in the face of Gauff's energy and electricity, and patiently waited for a chance to break back.
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She took that chance at 4-1 down as Gauff double faulted three times, and then Halep took full advantage of her opponent's wavering forehand to reel off the next four frames to take the match 6-4, 6-4 in just under 80 minutes and set up a quarter-final with Ons Jabeur.
Earlier on Monday, world no. 7 Jabeur - the highest ranked player left in the women's draw - battled through a brilliant encounter with Belinda Bencic to win 6-2, 3-6, 6-2 and book a place in the quarter-finals.
The two engaged in a series of titanic baseline rallies but Jabeur's dropshots were the highlight of the match, regularly catching Bencic off guard from the back of the court.
She also had the edge on Bencic's serve, breaking her twice in the final set for the win as the two stars embraced at the net.
The two had battled in April at the Charleston Open final, with Bencic coming out on top, so Jabeur was happy to get the better of her rival this time around.
"I wish I played like that in the final in Charleston," she said, admitting she was out for revenge in Madrid.
Jabeur also joked that she wanted Halep's match with Gauff to go the distance, and that she wanted the local crowd to be on her side after Halep knocked out Spaniard Paula Badosa in the round of 32.
"I hope Coco and Simona play for five hours," she said. "I saw Simona playing very well."
"Just a reminder, Simona won against Paula and she's Spanish so if she wins you have to cheer for me."
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