Three down after four holes, the World Number Two from Northern Irelands holed lengthy birdie putts on the sixth and eighth to draw level before a brilliant approach to within five feet at the ninth.
The winner-of the all-European contest will have a chance to go to World Number One later in the day if they can come the winner of the contest between two Americans, Hunter Mahan and Mark Wilson. With eight holes to play Mahan was two up.
Westwood wasted no time in taking control against his Ryder Cup teammate and former stablemate, showing unerring accuracy over the opening stretch to establish a commanding lead.
McIlroy chipped to a foot on the first to salvage a half, but the US Open Champion could not match Westwood’s birdie form four feet at the next.
The Northern Irishman found sand off the tee at the par three third, and despite a good chip missed his par putt from inside five feet.
Westwood was close enough to the green with his drive on the 393 yard fourth to use his putter for his second shot and then holed from five feet for a second birdie after McIlroy, in the rough off the tee, had missed from twice as far.
That made the gap three, but Westwood bogeyed the next after his approach ran through the green up against the collar of rough.
Westwood was the closer of the two on the short sixth, but McIlroy was the one to hole. A 28 foot birdie putt there gave him a second hole in a row after his former stablemate missed from 27 and there was only one in it once more.
There had been only two comebacks to win from three down in the first 60 games of the week, but McIlroy was back on terms by the long eighth.
He got much the better lie in the rough after he drove right and Westwood left, was just short of the green in two and made a 15 footer.
In the other semi-final Mahan led a topsy-turvy encounter.
Wilson drove into desert scrub at the first and Mahan’s third at the par five second finished inside four feet to put him two up.
But World Number 22 Mahan’s tee shot found the same bunker as McIlroy at the third and, although he pitched to eight feet at fourth and birdied, a shank at the fifth followed by a 20 footer from Wilson reduced Mahan’s lead to one up after five.
Mahan, though, returned to two up when his opponent missed the green at the seventh and had his second bogey of the day.
Mahan was still two up at the turn, pars for him and Wilson on eight and nine giving them outward halfs of 35 and 37, one under and one over, respectively.
McIlroy went two up with a 20 foot putt for yet another birdie on the short 12th, his fist-pump following one from Westwood on the previous green after the Englishman sank a nine footer for a half in birdie fours.
McIlroy had been really lucky there, hitting a wild second shot from the rough which was heading for the desert, but hit a cart path and rolled back onto grass by the green.
Wilson refused to let Mahan get away from him. He chipped in at the tenth after his opponent had hit his approach to within four feet and the 11th was shared in birdies again after he came out of sand to six feet and made it.
Mahan then bogeyed the 12th after playing a weak chip from over the green and Wilson might have levelled on the next, but missed from nine feet.
He hit two glorious woods onto the green, whereas Westwood could do no better than par after his drive went into the crowd and amazingly finished down the back of a lady's top.
Caddie Billy Foster jokingly asked her if she minded walking 250 yards down the fairways, but it was not a laughing matter for his player as he found himself in big trouble with only five to play.
