Published Monday, 03 September 2012
Louis Oosthuizen of South Africa shakes hands with Rory McIlroy. (© Getty)
The Northern Irishman started Sunday's third round one shot ahead of his playing partner and birdied the first two holes at TPC Boston.
But the South African also picked up a shot on the par-five second, before firing seven birdies in succession from the fourth.
Another birdie on the tenth gave Oosthuizen a tournament record 29 on the front nine, which he followed with six pars, a bogey, and a closing birdie to finish 19 under par.
McIlroy meanwhile returned a 67 to finish three behind on 16 under.
"Louis put on a display right there," he said.
"I was delighted when I got the honour back on the 12th tee. I've come from farther behind before - it's going to be an interesting day tomorrow."
Oosthuizen said: "Every putt had perfect speed. I told Rory, 'Sorry, but you've got to take it when you can."'
Dustin Johnston moved into third place on 13 under alongside Tiger Woods.