SHANGHAI, 2 Nov, 2014 (AFP) - France's Alexander Levy started the day five shots clear of Siem but was unable to cope with winds gusting to near gale force and slumped to a six-over par round of 78.
He finished tied for the lead on 16-under with Siem, who shot 73 on the par-72 Lake Malaren course, and England's Ross Fisher.
"It was really tough on the back nine," Siem told reporters. "The golf course was the total opposite of the first three days. A real monster, super brutal in the wind."
The round of the day belonged to Fisher, who shot a five-under 67 to claw back 11 shots on Levy and get into the playoff.
No one else in the 78-man field managed better than 69 Sunday as the wind bared its teeth after three days of low scoring in windless, soft conditions.
The trio trouped back to the 18th to go head-to-head into the fierce wind.
Both Fisher and Levy found the putting surface but Siem came up short in the thick greenside rough at the 471-yard closing par four.
But the German found the cup with his sand wedge to a huge roar from the galleries.
"I thought I had to hole the chip just to stay in the playoff," Siem said. "I was expecting one of the guys to hole their putt.
"A chip-in is always cool, but to have it in a playoff is even cooler."
Both Fisher and Levy then missed their own birdie putts and Siem could gratefully claim his fourth European Tour victory and his third via a playoff.
The German could have won it in regulation a few minutes earlier after Levy bogeyed the 18th.
But his eight-foot par putt for the win was pushed nervously right of the cup.
The victory and the winner's cheque for $1,166,000 propels the German to fourth in the Race to Dubai standings and gets him an entry into next week's World Golf Championships HSBC Champions events across the city in Sheshan.
Tied for fourth on 15-under par were Ryder Cup stars Jamie Donaldson and Justin Rose.
Donaldson started the day on 18-under par but fell back after finding the water at both the sixth and the 12th holes, eventually carding a 75.
"It's very difficult," said Donaldson of the back nine and the conditions. "Thirteen was brutal. The 16th I was hitting a three-iron in there today and it was a little nine-iron yesterday."
Rose returned a respectable level-par 72 in the conditions, but will probably cast his mind back to Thursday's first round when he carded a triple bogey eight at the straightforward par-five 13th, his fourth hole of the tournament.
Even a bogey that day would have meant he would have lifted the trophy on Sunday.
"It was a good fight. The week started terribly for me," said Rose.
"Four-over through four holes and hung in really well so I was proud to have any sort of chance coming up 18 today.
"It's fun that it came down to a nail-biting finish."
Final scores on Sunday after the fourth round of the European Tour’s $7 million BMW Masters at Lake Malaren Golf Club in Shanghai (par 72):
272 - *Marcel Siem (GER) 68-66-65-73, Ross Fisher (ENG) 70-67-68-67, Alexander Levy (FRA) 65-66-63-78
*Siem won at first playoff hole
273 - Justin Rose (ENG) 72-65-64-72, Jamie Donaldson (WAL) 68-68-62-75
274 - Romain Wattel (FRA) 66-67-71-70, Nicolas Colsaerts (BEL) 66-64-73-71
275 - Ryan Palmer (USA) 70-67-68-70, Emiliano Grillo (ARG) 66-68-69-72
276 - Fabrizio Zanotti (ITA) 70-68-69-69, Mikko Ilonen (FIN) 70-66-69-71, George Coetzee (RSA) 71-67-67-71
277 - Bernd Wiesberger (AUT) 72-69-66-70, Darren Fichardt (RSA) 69-68-69-71, Joost Luiten (NED) 72-69-64-72
278 - Richard Sterne (RSA) 68-70-70-70, Shane Lowry (IRL) 70-66-71-71, Branden Grace (RSA) 68-66-71-73, Robert Karlsson (SWE) 69-70-66-73, Graeme McDowell (NIR) 67-69-68-74
279 - Victor Dubuisson (FRA) 70-70-68-71, Danny Willett (ENG) 71-70-67-71, Richie Ramsay (SCO) 70-68-68-73