Vietnam’s former national champion was not in his peak form in the first-round match against Tortrakul, a player who advanced from the qualification round.
Thien lost a break point in the first set which saw Tortrakul win 6-4. He came back strongly in the second set winning it 6-3. It was a tough third set for both the players until the score reached 5-5, when Tortrakul stole a break point and sealed his win.
The Thai will next play No 8 seed Ryota Tanuma of Japan.
In other matches yesterday, No 6 Jeremy Beale struggled to win 6-7 (3), 6-7 (4) over Pol Wattanakul of Thailand; No 7 Denis Yevseyev of Kazakhstan beat Janie Asanee Praditngam of the United Kingdom; and No 4 Sidharth Rawat of India won 6-1, 6-1 over Socrates Leon Tsoronis from Australia.
Thiên will continue with the tournament in the doubles event.
He pairs with Ly Hoang Nam and will play Anthony Susanto and David Agung Susanto of Indonesia on June 21.