With Ian Poulter coming off second best to Westwood just as he did in practice on Tuesday, last season's Race to Dubai winner marched four clear of the field by going to the turn in a superb 33 and then making a curling 18 footer for his fourth birdie of the day at the tenth.
But then the excitement was turned up a notch when Mickelson holed a seven foot eagle putt at the long 13th - and then up several further levels when his 139 yard approach to the 14th spun into the hole for a two.
When Westwood failed to get up and down from a bunker on the 155 yard 'Golden Bell' 12th - the most famous par three in the whole of golf - the pair were level on 11 under par.
Poulter, like his European Ryder Cup team-mate chasing his first Major title, was having a real rollercoaster ride.
Joint leader with Westwood at halfway, the World Golf Championships-Accenture World Match Play champion bogeyed the fifth and sixth, birdied the next two, dropped another shot at the 11th and then double bogeyed the 12th after going into the same sand trap as his playing partner.
With that Poulter dropped to joint sixth on five under, six adrift.
Fifty year old Fred Couples, meanwhile, suddenly burst into third spot on eight under when he chipped in for an eagle of his own at the 530 yard 15th.
As for Tiger Woods, he was in a tie for fifth with fellow American Ricky Barnes after a real mixed bag of holes like Poulter.
He was only a stroke off the pace when he made an 18 footer on the first and a curling 25 footer at the third.
But the World Number One's swing started to let him down when he found sand on the fourth and then he three-putted the sixth and seventh for further bogeys.
He had failed to keep his emotions to himself after a poor tee shot on the sixth and, after grabbing a birdie at the eighth, he three-putted for a third time on the tenth.
At least he did not do it again three holes later and, having made the green in two, he returned to six under.
There was still time for at least one more switch in momentum, though, and it came when Mickelson three-putted the treacherous 17th and Westwood found the green in two on the 15th and made birdie to be one ahead again.