"I played well last week and took a lot from that. I knew that if I stayed patient I'd be right there at the end.
"I feel confident with the putter right now. It's so nice to get my hands on another trophy and get back in the winner's circle."
European Tour 2012 money list
1. Rory McIlroy (NI) - 3,407,300 euros
2. Peter Hanson (Swe) - 2,642,524 euros
3. Justin Rose (Eng) - 2,566,323 euros
4. Ian Poulter (Eng) - 2,270,851 euros
5. Branden Grace (SA) - 2,081,961 euros
Westwood led by three shots in the early stages but a double bogey at the short fifth stopped his momentum, before three bogeys on the back nine ended his hopes and he finished tied sixth with Oosthuizen.
With the overnight leaders out of contention, Mickelson, Els and defending champion Martin Kaymer were Poulter's biggest threats, only for the German to triple-bogey the short 17th.
Els, failed with a 45-foot birdie attempt at the last, with a bogey on the penultimate hole meaning Mickelson had to hole his second shot on the 18th to force a play-off.
The three-time Masters champion could only manage a par and now goes on to play at next week's Singapore Open.
Updated world rankings
1. Rory McIlroy (NI), 2. Tiger Woods (US), 3. Luke Donald (Eng), 4. Lee Westwood (Eng), 5. Justin Rose (Eng), 6. Adam Scott (Aus), 7. Jason Dufner (US), 8. Webb Simpson (US), 9. Brandt Snedeker (US), 10. Louis Oosthuizen (SA), 11. Bubba Watson (US), 12. Steve Stricker (US), 13. Phil Mickelson (US), 14. Keegan Bradley (US), 15. Ian Poulter (Eng), 16. Nick Watney (US), 17. Matt Kuchar (US), 18. Dustin Johnson (US), 19. Peter Hanson (Swe), 20. Ernie Els (SA)
"I'll see if I can improve one spot there," he said. "It was good for me to come back after having not played since the Ryder Cup and pick up where I left off."
Another man who will be in Singapore is Rory McIlroy, who along with Tiger Woods,
skipped Mission Hills
but holds a 800,000 euro lead at the top of the European Tour money list after nearest challengers Justin Rose and Peter Hanson finished in a tie for 24th.
Poulter, only the second European after Darren Clarke to win two WGC events, has climbed to fourth in the Race to Dubai and 15 in the world rankings.
"Two world championships are great, of course, but a major is the biggest dream," he concluded. "People keep asking all the time 'when, when, when'. I don't know when and I'm trying really hard - I'll do my best next year."
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