The 2016 champion appeared set for an expected easy victory after restricting Thailand to a modest 78 for nine off its 20 overs.
But the West Indies struggled at 27 for3 in the chase before captain Stafanie Taylor (26 not out) and Shemaine Campbelle (25 not out) chased down the total with 20 deliveries to spare.
Naruemol Chaiwai’s spectacular run out of opener Lee-Ann Kirby for three highlighted Thailand’s impressive fielding performance.
Later, Sophie Devine’s sixth consecutive 50-plus score steered New Zealand to victory against Sri Lanka.
It took time for New Zealand to get the 128-run chase going, but captain Devine led by example, becoming the only player, male or female, to register more than six consecutive 50-plus scores in the format, en route to the seven-wicket victory.
Brief scores: Thailand 78/9 lost to West Indies 80/3 in 16.4 overs (S Taylor 26*); Sri Lanka 127/7 in 20 overs lost to New Zealand 131/3 in 17.4 overs (S Devine 75*)