Westwood, who set a record at the Gary Player Country Club with a 10-under-par 62 Saturday, shot 1-over-par 73 Sunday and won the tournament by two strokes at 15-under. He won the select 12-player event last year with a score of 17-under.
The 62 on Saturday gave Westwood a 7-stroke lead going into the final round and, even though Westwood's Sunday round ended with back-to-back bogeys, his lead was so comfortable at that point he still collected the victory.
Robert Karlsson shot 4-under 68 Sunday and was the only player in the field to complete all four rounds less than 70. He wound up second at 13-under. Jason Dufner and Graeme McDowell, who each shot 2-under 70 Sunday, tied for third at 11-under. Kim Kyung-tae, with four consecutive rounds of 70, was fifth at 8-under.
Westwood opened Sunday's play with five straight pars. He had birdies at Nos. 6 and 9 but toured the second nine in 3-over, with a bogey at 12 before the bogeys at 17 and 18.
Charl Schwartzel (72 Sunday) finished in sixth at 6-under while Luke Donald (72) was seventh at 5-under and Martin Kaymer (76) eighth another stroke back.
Simon Dyson was even-par in the final round and ended up in ninth at 1-under. Andreas Hansen (70) was even for the tournament while Francesco Molinari (73) was 11th at 7-over and Darren Clarke (78), 12th.
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