The Polish player, seeded fifth, dominated from the start against fourth-seeded Mertens, who was playing her first match of the week after receiving a bye in the opening round and a walkover in the second when Barbora Krejcikova withdrew.
Mertens, ranked 19th in the world, struggled to find her rhythm throughout, while Chwalińska played with confidence and variety to seal the win.
It was the Pole's third straight-sets victory of the tournament, following wins over Rebecca Sramkova of Slovakia and Oleksandra Oliinykova of Ukraine.
The result echoed her win over Mertens at this year's French Open, where she beat the Belgian en route to a career-best run to the final.
Chwalińska will next face Leylah Fernandez, who beat top seed Mirra Andreeva 6-2, 7-5 in the other quarter-final.
Regardless of how the tournament ends, the Polish player is set to rise from 25th to 21st in the WTA rankings, matching her career-high position.
The Singapore Tennis Open takes place from 21 to 27 September.
(ał)
Source: PAP