Mickelson survived some tough moments on the back nine to win
-10* P Mickelson (US), R Fisher (Eng), L Westwood (Eng)
-9 K Stadler (US)
-8 P Harrington (Ire), V Singh (Fij)
-7 S Webster (Eng)
Even R Goosen (SA)
+1 E Els (SA)
+6 I Poulter (Eng)
+7 S Garcia (Spa)
* Mickelson wins at 2nd extra hole
Phil Mickelson was taken to a second extra hole in a play-off with English duo Ross Fisher and Lee Westwood before winning the HSBC Champions in Shanghai.
The American dropped four shots in as many holes on the back nine to almost blow his chances, but playing partner Fisher took a seven at the last.
Mickelson's six at the same hole was enough to force the play-off with Westwood, all three on 10-under.
And in near darkness a Mickelson chip and putt for birdie won him the title.
"It feels amazing," said Mickelson. "I don't how it happened, the whole day was a whirlwind.
"I had to fight it out because I had a big lead but then hit some horrible shots. It's a funny game, golf, because I thought I was out of it at the last when I hit the water."
Mickelson had begun the day in pole position, two clear of nearest challenger Fisher.
And after 11 holes he remained on his overnight score of 16-under, well in control of affairs.
But he dropped four shots between the 12th and 15th to open the door for Fisher, whose birdie at the 13th also helped swing things in his favour.
Going down the par five 18th, the Englishman suddenly had a one-shot lead over the world number two and both hit good drives.
Fisher was first to play his second shot, and took the bold option of going for the green.
Taking a wood, he pulled the shot a fraction to finish on the greenside rough, while Mickelson found water.
Looking for all the world a winner, Fisher contrived to hit his next shot past the flag and into water, leaving Mickelson a put for a par and the tournament.
Fisher took seven after almost reaching the 18th in two shots
He missed that, before Fisher bravely holed an eight-footer for a seven to join his adversary in the play-off.
It was extraordinary that Westwood had reached the play-off after being 12 shots behind the lead at the turn on Sunday.
But he strung together six birdies on the back nine to roar up the leaderboard.
On the first extra hole, where the players took on the 18th once again, all three had birdie putts to win.
Westwood and Mickelson each missed 20-footers while Fisher had a shorter putt, but from the fringe.
All missed, and they played the hole again.
Westwood quickly dropped out of contention, however, and when Fisher missed a 12-foot birdie putt, Mickelson sunk his from four feet to win.
Paul Casey, playing in the same group as Fisher and Mickelson after starting the day on 11-under, fell away with a horrible 78, 12 shots worse than his immaculate third round.
His demise left Friday's leader Kevin Stadler to secure fourth place on nine-under, with Padraig Harrington and Vijay Singh sharing fifth, a further shot behind.