World number three Simona Halep was dumped out of the Madrid Open on Tuesday, losing 4-6, 7-5, 7-5 in the last 16 to Belgian Elise Mertens.
Romania's Halep, who won the Madrid title in 2016 and 2017, is still yet to reach a WTA Tour final this season and saw her earliest exit in the Spanish capital since a 2015 first-round loss.
Halep had sailed through the first two rounds on the Madrid clay arena without dropping a set but she ended up second best in a match which yielded 15 breaks of serve.
Halep served 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 her doubles partner, 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 Spaniard Paula Bad
Osa, ranked 62 in the world.
