Sudaporn Seesondee edged Great Britain's Caroline Dubois 3-2 in the women's boxing lightweight (60kg) quarter-finals at the Tokyo Games on Tuesday.
The Thai boxer won the first round but Dubois came back to even the match in the second round. The Udon Thani native powered past her opponent in the deciding bout to close the contest 3-2 in a nail-biting match.
The win guaranteed her at least a bronze medal. She will be the first Thai women's boxer to get an Olympic medal.
Her march to a semi-final spot will earn her a position as a civil servant in the Royal Thai Navy.
She will meet Kellie Harrington from Ireland in the semi-finals.
Sudaporn, a silver medallist at the 2018 world championships, comfortably beat Simranjit Kaur of India 5-0 in the round of 16.
The Thai team have so far won only one medal. Taekwondo champion Panipak Wongpattanakit took gold in the women's 49kg event .