The 30-year-old Belgian, ranked 19th in the world, produced a composed display to secure back-to-back victories over Grand Slam champions and advance to just the third WTA 1000 semi-final of her career and her first since 2021.
Mertens seized control by dominating the opening-set tiebreak without dropping a point and carried that momentum into the second set, breaking Swiatek and completing one of the biggest upsets of the tournament.
She will next face home favourite Zheng Qinwen, who defeated sixth seed Elina Svitolina 6-3 7-6 (8-6) earlier in the day to book her place in the last four.
Playing on a wildcard, Zheng took charge early by breaking Svitolina three times in the opening set. The Ukrainian world No.7 responded strongly in the second, raising her level to force a tiebreak.
Zheng recovered from a sluggish start in the breaker, reeling off four consecutive points to regain the initiative before sealing victory in one hour and 47 minutes.
Svitolina competed well throughout but was unable to withstand Zheng's attacking game as the Olympic champion consistently forced the pace and seized the initiative in crucial moments.
"I'm just happy to have a lot of winners during the match, and at the end, be brave in important moments," Zheng said.
The victory took Zheng to her fourth career WTA 1000 semi-final and her third on Chinese soil, following appearances in Beijing and Wuhan in 2024.
