Mai Hontama stunned top seed Dayana Yastremska in the first round of the Allied Thailand Open at Arena Hua Hin on Tuesday.
Japan's Hontama edged Ukraine's Yastremska, the 2019 champion, 7-6 (7/4), 7-5 to set up a second round clash with Thai wildcard Lanlana Tararudee.
Earlier, Dutch qualifier Arianne Hartono knocked out seventh seed Moyuka Uchijima of Japan in another first round upset of the WTA 250 tournament.
Hartono produced an impressive performance as she brushed aside the Japanese star 6-1, 6-1 to secure her place in the second round against Gao Xinyu of China.
Hartono cruised to a comfortable win in the first set, dropping just one game.
The 59th-ranked Uchijima tried to fight back but was undone by eight double faults as Hartono took the second set by the same scoreline.
World No.185 Gao, meanwhile, defeated the 151st-ranked compatriot Wei Sijia 6-3, 6-2 to progress.
Argentine world No.74 Nadia Podoroska also advanced to the second round after defeating Russian qualifier Ksenia Laskutova 6-1, 6-0.
Podoroska will face second seed Katerina Siniakova of the Czech Republic in the second round.
Also on Tuesday, Jana Fett of Croatia rallied to beat Zheng Saisai of China 2-6, 6-3, 6-3.
Fett will play Varvara Gracheva of France in the second round.