Halep, the 2016 and 2017 winner of the clay-court tournament, made the stronger start to proceedings on the Manolo Santana by taking the opening set 6-4.

The Romanian star proved unable to seal victory in a match dominated by breaks of serve, however.

Mertens was able to win the second set 7-5 to force a decider, where she triumphed by the same score.

Halep was left ruing being broken 10 times during the match, having also broken her opponent on nine occasions.

Mertens will now face Belarus’ Aryna Sabalenka in the quarter-finals of the Women’s Tennis Association (WTA) Premier 1000 event.

Russia's Anastasia Pavlyuchenkova is also through to the last eight after overcoming the United States’ Jennifer Brady 7-5, 6-7 (8-10), 6-3.

The hard-fought match lasted 2 hours 25 minutes.

Pavlyuchenkova will face either Greece’s Maria Sakkari or the Czech Republic’s Karolína Muchová, with the latter looking to build on her win over second seed Naomi Osaka of Japan.

The men’s tournament continued with a mix of first- and second-round matches.

Japan's Kei Nishikori earned a three-set 6-7 (6-8), 6-2, 6-2 win over Russia's Karan Khachanov to set up a second-round match with Germany’s Alexander Zverev, the tournament fifth seed.

Spain’s Roberto Bautista Agut also advanced in three sets, overcoming Italy’s Marco Cecchinato 6-2, 6-7 (3-7), 7-5.

He will now face American John Isner.

Russia's Alexander Rublev was among the first players to advance to the third round, as the sixth seed came from behind to beat his American opponent Tommy Paul 6-7 (7-5),6-3, 6-4.

Matteo Berrettini secured victory in an all-Italian second-round tie, overcoming compatriot Fabio Fognini in straight sets 6-3, 6-4.