Halep converted her fifth match point to win her 14th straight match in Madrid and stay on track for a third consecutive title in the Spanish capital.

"I knew she was coming with a lot of confidence," Halep said. "I had to start very strong and aggressive, which I did. After I played a few games I felt really good."

The 16th-ranked Mertens has played well this year but struggled from the start at the Magic Box and ended up with 31 unforced errors and five double faults.

She was coming off an impressive 13-match winning streak, with titles in Morocco, Switzerland and Tasmania. She also reached the Australian Open semifinals, losing to eventual champion Caroline Wozniacki.

Halep next faces Kristyna Pliskova, who defeated Sara Sorribes Tormo 7-5, 6-2.

"She's serving pretty strong," said Halep, who beat Pliskova in Madrid in 2017. "I have to just watch a little bit how I played last year and to do it again. I believe I can win the match but I always expect a tough one."

Petra Kvitova defeated Monica Puig 6-3, 7-6 (8) to record her seventh straight win.

In the first round of the men's event, Robin Haase ended a four-match losing streak by beating Hyeon Chung 6-2, 6-0.

Denis Shapovalov hit 38 winners to defeat Benoit Paire 7-6 (5), 4-6, 6-4 in the second round while 11th seed Roberto Bautista Agut beat Jared Donaldson 6-7 (3), 6-4, 6-4 in the first round.

Borna Coric upset ninth seed Pablo Carreno Busta 6-4, 6-2 in another first-round match.

Juan Martin del Potro marked his clay-season debut with a 6-3, 6-3 win over Damir Dzumhur in their second-round match.

It was the 22nd triumph this season for the sixth-ranked del Potro, who has the most victories on the ATP Tour in 2018.

"I was playing well out there,'' said del Potro, who won back-to-back titles in Acapulco and Indian Wells earlier this year.

Del Potro's next match is against Dusan Lajovic, who defeated Richard Gasquet 7-6 (1), 7-6 (1).