By Gabriel Tan at the Jalan Besar Stadium
Radojko Avramovic's men got off to a dream start as Mustafic Fahrudin fired them ahead from the spot after ten minutes, but the War Elephants equalised in the 59th minute when Adul Lahsoh grabbed a valuable away goal.
Nonetheless, it took the Lions just three minutes to reclaim the lead when Khairul Amri netted his third goal of the tournament, before Baihakki Khaizan ventured forward and scored from close range to give his side a strong advantage ahead of Saturday's second leg.
Five minutes in, Singapore carved open the opposition defence for the first time when a neat one-two between Amri and Shahril Ishak saw the latter burst into the box, only for Chonlatit Jantakam to weigh in with a good sliding block.
However, they were presented a golden opportunity to open the scoring three minutes later when Aleksandar Duric was hauled down by Piyaphon Buntao inside the area as he looked to latch onto a cross by Shahril, leaving Toma Masaaki no choice but to point to the spot.
Mustafic, who also netted from the spot in the 2007 final, stepped up and coolly wrongfooted Kawin Thamsatchanan to send the Jalan Besar faithful into joyous celebrations, but those were cut short as the Japanese referee ordered a retake.
Showing no signs of nerves, the Lions anchorman steadied himself a second time before sending the opposition keeper the wrong way again to put his side ahead.
Kawin then had to be alert to prevent the hosts from doubling their tally in the 15th minute after Amri decided to try his luck from 30 yards with a cross-cum-shot that looked set to dip just beneath the bar.
By now, Singapore were starting to control the game as they denied their opponents any space and time on the ball with their aggressive pressing game, and it took the visitors until the half-hour mark before they created their first real chance - Anucha Kitpongsri screwing a shot wide of the far post after a lovely through-pass by Phichitphong Choeichiu.
A minute before the break, the War Elephants came close to carving out another opening when Datsakorn Thonglao's visionary pass released Teerasil Dangda down the right. The Muangthong United star advanced into the box before playing a neat ball across goal that looked set to pick out the rampaging Phichitphong until a timely intervention from Daniel Bennett.
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Four minutes into the second half, Teerasil wasted a good chance to equalise when he sent a free header over after a searching cross from Piyaphon, who had also assisted the striker in the semi-final first leg against Malaysia.
But there was to be no denying Thailand a minute before the hour mark as they finally pulled level thanks a terrific piece of play from Adul. After winning possession in the middle of the park, the Chonburi man sent Teerasil racing down the wing before charging forward in support, and was duly rewarded for his endeavour when his team-mate returned the ball to him to stab home off the post from six yards out.
Although Adul's strike reduced the home fans to a stunned silence, they only had to wait three minutes before they were jumping for joy once more when Amri received possession from Duric and created space for himself with an ingenious spin, before drilling a shot past a hapless Kawin.
The visitors nearly mustered an immediate reply of their own a minute later when Jakkapan Pornsai tried his luck with a freekick from 30 yards, only to see his ferocious effort come back off the bar.
Still, it was the Lions who looked the more dangerous of the two sides and Shahril could have doubled their lead in the 73rd minute when he latched onto a loose ball on the edge of the area and skipped past the last defender in Theerathon Bunmathan, but Kawin was quick to race off his line and managed to smother at the Singapore captain's feet.
As the game entered its closing stages, Thailand again had their goalkeeper to thank as he pulled off flying saves to keep out a pair of blistering freekicks from Shaiful Esah.
But a minute into injury time, Singapore grabbed extra insurance when they added a third after Shahril missed an attempted header from a Shaiful cross at the back post but managed to divert the ball back into the six-yard box, where Baihakki proceeded to stab home from close range.
And while there's still 90 minutes before this year's Suzuki Cup champions are crowned, there's no doubting the Lions are in pole position to do just that heading into the return encounter at the Suphachalasai Stadium in three days' time.
Singapore: Izwan Mahbud, Daniel Bennett, Baihakki Khaizan, Safuwan Baharudin (Irwan Shah 90'), Shaiful Esah, Shahdan Sulaiman, Mustafic Fahrudin, Isa Halim, Aleksandar Duric, Shahril Ishak, Khairul Amri (Firdaus Kasman 87').
Thailand: Kawin Thamsatchanan, Piyaphon Buntao, Panupong Wongsa (Nataporn Phanrit 27'), Chonlatit Jantakam, Theerathon Bunmathan, Adul Lahsoh, Phichitphong Choeichiu, Datsakorn Thonglao (Pipob On-mo 79'), Jakkapan Pornsai, Anucha Kitpongsri (Arthit Sunthornpit 54'), Teerasil Dangda.