Thailand took a 2-0 lead against Bolivia on the first day of their Davis Cup World Group II clash at the National Tennis Development Centre at Muang Thong Thani on Friday.
Thailand No.1 Kasidit Samrej comfortably defeated Bolivia's Federico Zeballos 6-3, 6-2 in the opening singles match to give the home team a 1-0 advantage.
In the second singles, Thailand's world No.960 Yuttana Charoenphon upset No.361 Murkel Dellien 2-1.
Yuttana won the first set 6-2 but lost 6-0 in the second.
Murkel led 5-3 in the deciding set but the Thai fought back to make it 6-6.
Yuttana then won the tie-break 8-6.
Thailand now need just one win to reach the World Group play-offs.
"Before the match, I tried to forget my poor record in the Davis Cup and it worked," Kasidit said.
"He did not hit the ball very hard and that allowed me to play easier."
Yuttana said: "I did not want to lose so I gave my all."
Thailand captain Paradorn Srichaphan said he was confident that his men would be able to seal one more point to win the tie.
Today's action begins at 11am with the doubles match between Thailand's Prachya Isaro/Thantub Suksumrarn and Bolivia's Boris Arias/Zeballos.
It will be followed by Kasidit v Dellien and Yuttana v Zeballos in the reverse singles matches.