The three-time champions, twice losing finalists since last lifting the trophy in 2002, edged the Philippines 2-1, thumped qualifiers Myanmar 4-0 and then dismissed Vietnam 3-1 during their Group A games in Bangkok.
Next up is a difficult, two-legged clash against fellow co-hosts Malaysia, who recovered from an opening 3-0 loss against Singapore to beat Laos 4-1 and Indonesia 2-0 to reach the last four as Group B runners-up.
Singapore, three-time winners of the biennial Asean Football Federation (AFF) championship, face the rapidly improving Philippines - featuring ex-Chelsea poster boys Phil and James Younghusband - in the other home-and-away semi.
Thailand start as favourites against Malaysia after beating them 2-0 in a friendly last month, but they will be wary of the Tigers, who host the first leg at Kuala Lumpur's cacophonous Bukit Jalil stadium on Sunday.
"They're dangerous, especially at home where passionate fans normally pack the stadium to the rafters. Such strong support really helps them as the players feel more motivated," said Thailand's coach Winfried Schaefer.
"We beat them 2-0 recently but we cannot read much into that result because both of us used many reserves in that game. The win was no doubt a boost to our morale, but we cannot underestimate them.
"They pass the ball well and their teamwork is good since they have been together for a while."
After resting players against Vietnam, Thailand are expected to be at full strength including striker Teerasil Dangda and midfielder Datsakorn Thonglao, who went off with a groin problem against the Philippines.
The same cannot be said of Malaysia, who have five players on the injury list including winger Wan Zack Haikal Wan Nor and midfielder Shakir Shaari.
"This puts us in a difficult situation ahead of our match against Thailand," said coach K. Rajagobal. "We still have few more days to prepare for the match. I'm keeping my fingers crossed and hoping for the best for Shakir and Zack."
Manilla plays host for emotional semi-final
In Manila the Philippines, denied the chance to host a semi-final leg two years ago due to the lack of a suitable venue, will finally play in front of their home fans at the renovated Rizal Memorial Stadium on Saturday.
"Now that we are here, we are hoping the home fans can help get us through," said team manager Dan Palami.
The encounter will be emotional for the Azkals after hundreds of people died and 300,000 were made homeless when a typhoon hit the south of the country this week.
Singapore topped Group B but they will be missing midfielder Hariss Harun, out for the rest of the tournament with a shin fracture, and winger Shi Jiayi, who has travelled to Shanghai for a family emergency.