The German holed out from about 30 feet (10 metres) for birdie at the first extra hole to edge Frenchman Alexander Levy and Briton Ross Fisher after the trio had finished locked at 16-under-par 272 at Lake Malaren.
Siem, 34, claimed his third victory on the European Tour, coming from five strokes behind overnight leader Levy with a one-over 73 in strong northerly winds that blew in behind a cold front that swept in late morning and blew away a pesky layer of clouds that had hung over the city for several days.
Levy hit a series of poor shots and nervous putts to card 78, while Fisher’s superb 67 was the best round of the day as he overcame an 11-shot deficit to almost pull off what would have been a stunning victory.
Siem’s win earned him a late invitation into this week’s WGC HSBC Champions event at nearby Sheshan Golf Club, which means he must cancel a holiday with his wife in Phuket, Thailand.
“It would have been the first time (in) five years on a holiday on our own,” said Siem, whose wife was already scheduled to arrive in Thailand by the time he spoke to the media.
Instead, he must try to arrange a visa for her to come to Shanghai but he was complaining.
Siem, who has had two knee surgeries this year, overtook Levy on a day when the gusty winds made yardage books almost useless and sent scores soaring after three days of ideal scoring conditions.
“The course was total opposite to the first three days — a real monster, and I’m super pleased,” he said. “I still can’t believe it. I’m over the moon. I came here with very, very low expectations because of my two surgeries.”
But he almost squandered his chance of winning by bogeying the final two holes of regulation, missing a five-footer at the last that did not even touch the cup, though he would make amends at the same hole minutes later.
Levy, 40-under during his previous five strokeplay rounds, came back to reality with a thud.
“Today was not very easy for me. I’ve got to work on that and be more strong during difficult weather like that,” he said.
“He (Siem) deserved to win because he played fantastic in the wind.”
Levy was far from the only player to struggle. Eight players did not break 80, including Spaniard Miguel Angel Jimenez, whose 88 included a 13 at the water-lined par-four ninth.
The tournament was the first of four events in the European Tour’s Final Series.
(Editing by John O’Brien/Sudipto Ganguly)