Second seed Mertens cruised to a 6-2 6-4 win over Slovenian Polona Hercog and is on course to win her fourth title of the year after triumphs at Hobart, Lugano and Rabat.
Charleston Open winner and third seed Kiki Bertens had a tougher match against Russian Natalia Vikhlyantseva as the world number 19 earned a hard-fought 7-5 3-6 7-6(5) win over her 21-year-old opponent.
China's Zhang Shuai, who has singles ranking of 29, was the only other seeded player to exit the tournament after a 6-3 6-3 defeat by 19-year-old Hungarian Fanny Stollar.
American Coco Vandeweghe, who won the title in 2016, beat Belarusian Aliaksandra Sasnovich in straight sets earlier in the tournament to keep her hopes of reclaiming the trophy alive.
The 26-year-old will face Arantxa Rus in the last-16 on Wednesday while Belgian sixth-seed Alison Van Uytvanck and Serbian Aleksandra Krunic will also be in action.
(Reporting by Aditi Prakash in Bengaluru; Editing by Christian Radnedge)
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