This is the best match on the Singapore card and the odds agree. Mertens is 1.77, Krejcikova 2.04, which is about as close as a second-round line gets at a WTA 500.
Mertens is the defending champion, seeded seventh at No. 19 in the world, and she won the title here when the event was a February 250. The upgrade to 500 status and the move to September has not changed the venue or the surface, and familiarity with an indoor court is worth more than it sounds. Mertens does not overpower anyone, but her serve placement and her ability to hold rally length are well suited to a building where the ball comes through at a predictable height.
Krejcikova leads nothing in this matchup. Mertens took their most recent meeting 5-7 6-1 7-5 in the Australian quarterfinals in January, a match that turned on Krejcikova's inability to close from ahead. That is the pattern to watch. Krejcikova at her best hits through Mertens comfortably. Krejcikova over three sets, with her injury record and her service rhythm coming and going, is a different proposition.
Expect a first set decided by a single break, and a third set that rewards whoever is still serving cleanly after two hours.
Mertens in three.