By Marcus Chhan
The Italian teenager and the major winner from South Africa went into the sudden death play-off at 13 under par after shooting 69 and 67 respectively.
Manassero looked in pole position to win the title at Sentosa's Serapong course after chipping to within a few feet on the first playoff hole at the par-five 18th. However, Oosthuizen showed those famous nerves of steel and drained an eight foot putt for birdie to ensure his Italian opponent's subsequent tap-in would only extend the contest further.
Back to the 18th tee the two men went, and more drama was to unfold as Oosthuizen's tee shot found the bunker while Manassero the fairway. After laying up, Oosthuizen managed to stick his third shot five feet from the hole. Manassero failed to capitalise anyways and after playing his third shot he was still 15ft away from the hole - he would miss the birdie putt and tap-in for par.
This left the door wide open for Oosthuizen to win the tournament from just five feet but his putt somehow lipped out of the cup.
On the third playoff hole, Oosthuizen must have thought he caught a break, though, as his second shot at the par-five 18th trickled down onto the green while Manassero's landed 12ft from the pin.
But the South African could not hole his eagle putt leaving the 19-year-old Italian to show his class by draining the pressure putt from 12ft for eagle and collect the US$1million winners' cheque.
The victory was Manassero's third European Tour International Schedule win and he is now also the youngest winner of the Barclays Singapore Open at 19 years and 206 days.
"It has been an extremely long day," Manassero said.
"The adrenaline kept me going and fortunately the key was starting very well this morning [Sunday] after coming back on the course and I started with some birdies in a row [during the third round] which was very good.
"And the play off was just a great honour to play with Louis. It is the first playoff of my career and it has just been amazing really. Ending with an eagle was great.
"Of course [I thought I had lost it on the second play off hole], Louis had a really near chance. You kind of think I have almost lost it. I did not have a really good line - I was a little bit unfortunate with the second shot and did not have a good lie to make birdie and he had that chance."
Meanwhile, world number one Rory McIlroy roared through to third place at Sentosa with a closing 30-foot eagle putt for a 65.
He finished two behind Manassero and Oosthuizen in regulation play, and in the process became the European Tour's youngest money list winner since Sandy Lyle in 1980.
Already with the PGA Tour title in the bag, the 23-year-old world number one did the double when Manassero beat Oosthuizen at the third play-off hole.
Fourth place went to Thomas Bjorn while Italian Francesco Molinari and Australian Adam Scott were in joint fifth.