Chanathip Songkrasin scored twice as Thailand beat defending champions Vietnam 2-0 in the first leg of their AFF Suzuki Cup semi-final at the National Stadium in Singapore on Thursday.
The Thai skipper capitalised on a defensive error to give the five-time winners the lead after 14 minutes.
Nguyen Phong Hong Duy slipped as he attempted to cut off a pass, allowing Chanathip to race through and clip the ball past Tran Nguyen Manh.
Nine minutes after his fortunate opener, he added a sublime second.
Chanathip exchanged a series of quick passes with Teerasil Dangda and Sarach Yooyan to carve open the Vietnamese defence before coolly slotting home from the edge of the box.
Vietnam, who had not conceded a goal in their four group games, hit the woodwork twice.
In the first half, Nguyen Quang Hai's thunderous freekick from 25 metres cannoned off the right post.
And he was denied again in the second half when his left-footed effort from the edge of the box hit the crossbar.
Chanathip had a chance to wrap up the tie and complete his hat-trick five minutes from time when Thailand were awarded a penalty, but he failed to beat the Vietnam keeper.
Vietnam will now look to turn things around in the second leg on Sunday, to be played at the same venue.
The winners will meet Indonesia or hosts Singapore in the final next week.