MANILA, Philippines — World No. 1 and defending champion Iga Swiatek reached the quarterfinals of the WTA Stuttgart clay-court tournament by beating Elise Mertens on Thursday, her ninth win in nine matches at the French Open warm-up event. Swiatek beat the unseeded Belgian 6-3, 6-4 to maintain her bid to capture a third successive title and a third luxury car from the sponsors after also winning the tournament in 2022. She faces former US Open champion Emma Raducanu for a place in the semifinals.