World number three Simona Halep was dumped out of the Madrid Open on Tuesday, losing 4-6, 7-5, 7-5 in the round of 16 to the Belgian Elise Mertens.
Halep had sailed through the first two rounds on the Spanish clay without dropping a set but she ended up second best in a match which yielded 15 breaks of serve.
Halep produced 10 double faults in the match, Mertens nine.
Halep, a two-time Grand Slam champion and former world number one, was twice a break up in the final set at 3-1 and 4-3 but finally succumbed after just over two and a half hours of play.
In the quarter-finals 13th seed Mertens will meet the Belarusian seventh seed Aryna Sabalenka, who dispatched American Jessica Pegula 6-1, 6-2 in just 52 minutes.
Two other quarter-final line-ups are also settled with world number one Ashleigh Barty set to meet ninth seed Petra Kvitova while eighth seed Belinda Bencic is up against unseeded Spaniard Paula Badosa, ranked 62 in the world.