Defending champion Swiatek motors into Stuttgart quarter-finals
Swiatek beat the unseeded Belgian 6-3, 6-4 to maintain her bid to capture a third successive title in the German city and be handed the keys to a third luxury car from the sponsors after also winning the tournament in 2022.
"There's always space for a Porsche. If not, we'll make it. I'll build an underground garage," said Swiatek after playing her first clay-court match since lifting the French Open title last June.
The Pole burst out of the blocks to set up a 5-1 lead in the first set, before going 0-30 down but recovering to serve out the set.
Swiatek was broken early in the second but served her way back into the set, winning with a forehand on her fourth match point after Mertens had saved the previous three.
"This is not an easy tournament. Everyone is really motivated to win that car," Swiatek added.
The top seed will face former US Open champion Emma Raducanu for a place in the semi-finals.
Raducanu, who helped Britain qualify for the Billie Jean King Cup finals last weekend, took down Czech teenager Linda Noskova 6-0, 7-5.
It will be a first quarter-final appearance for Raducanu in 19 months. She has been plagued by a raft of injuries since her 2021 Grand Slam breakthrough and missed much of last season.
Ukraine's Marta Kostyuk saved five match points at 5-4 in the final set to defeat fifth seed Zheng Qinwen of China, 6-2, 4-6, 7-5.
World number 27 Kostyuk will face US Open champion Coco Gauff on Friday for a place in the semi-finals.
Elena Rybakina beat Veronika Kudermetova 7-6 (7/3), 1-6, 6-4, continuing her strong form in 2024.
The Kazakh world number four, who has already claimed titles at Brisbane and Abu Dhabi this year, beat the Russian in two hours 33 minutes.
"I know I have my weapon, my serve. I know I can always serve it out in tough moments. Not always, but this is a strength," Rybakina said.
Rybakina will be playing in her seventh quarter-final of the season on Friday where she will face Jasmine Paolini who put out Ons Jabeur 7-6 (10/8), 6-4.
Wimbledon champion Marketa Vondrousova defeated Anastasia Potapova 7-6 (7/5), 6-1 in her last 16 clash.
