Amanda Anisimova celebrated her fifth career top-10 win by beating Aryna Sabalenka again at the Madrid Open.

The 20-year-old American would have headed into Thursday's match-up full of confidence having also defeated the world number four in Charleston earlier this month.

And again Anisimova came out on top, this time at a tournament where Sabalenka was the defending champion.

Sabalenka is now 0-4 against Anisimova, losing three times on clay, with this the underdog's first ever victory in Madrid.

A 6-2 3-6 6-4 triumph was sealed in an hour and 55 minutes, with Anisimova relishing the opportunity to take on one of the sport's foremost stars once more.

"It's always enjoyable to accept the challenge, even when it's a tough match, and push yourself and see how far you can go," Anisimova said.

"I enjoy these matches, even though they're very tough – especially when you get to win them."

While Anisimova can look forward to facing Petra Martic in round two, hers was not the only upset win, with Karolina Pliskova and Jelena Ostapenko also ousted.

Pliskova lost in straight sets to fellow Czech Marie Bouzkova, as Ostapenko went down in three to Ekaterina Alexandrova.

Simona Halep and next opponent Paula Badosa both came through unscathed, though, as did Belinda Bencic, Coco Gauff and Ons Jabeur.

Jabeur's 11-9 first-set tie-break success against Jasmine Paolini was the closest any of that quintet came to dropping a set on day one.