Thailand's U23 team lost to South Korea 1-0 in their Group B match at the 5th WAFF U23 Championship in Saudi Arabia on Wednesday night.
Cho Hyun-Taek scored the winning goal for South Korea in first half injury-time at Al Fateh SC Stadium in Al Mubarraz.
Thailand will face Jordan, who lost to hosts Saudi Arabia in Group A, in the classification semi-finals tomorrow.
South Korea and Saudi Arabia will meet in the semi-finals on the same day.
South Korea dominated the first half and put a lot of pressure on Issara Sritaro's team but couldn't find the back of the net as the match headed into the interval.
Chon Hyun-Taek, set up by Kang Hyun-Muk, fired a shot through the outstretched hands of Thailand goalkeeper Thirawooth Sraunson in the second minute of first-half injury time as the Koreans got the goal they were looking for.
Thailand had a promising opportunity with Sittha Boonla but his shot missed the target.
The Koreans still looked strong in the second half and nearly doubled their lead but a shot from Yang Hyun-Jun was denied by Thirawooth.
Natcha Promsomboon came close to pull Thailand level 10 minutes from time but his header went wide.
Jordan lost to hosts Saudi Arabia 2-1 in the penalty shoot-out after a 1-1 draw in Group A.
In other matches on Wednesday, Egypt defeated United Arab Emirates 1-0 in Group C while Australia edged Iraq 2-1 in Group D.
Egypt and Australia will face off in the semi-final while Iraq and UAE will play in the classification semi-finals.
Thailand have been invited to take part in the March 20-26 tournament by the West Asian Football Federation (WAFF).
The tournament will serve as a warm-up for the AFC U23 Asian Cup in Qatar, starting April 15.