Rory McIlroy quickly made major inroads into Sergio Garcia's lead when the final pair belatedly began their closing rounds in the WGC-Bridgestone Invitational on Sunday.
Torrential rain which forced play to be suspended earlier in the day meant the Ryder Cup team-mates did not tee off until 3:20pm local time at Firestone Country Club, 75 minutes later than scheduled.
But McIlroy wasted no time in reducing a three-shot deficit to just a single stroke with birdies on the first two holes as he sought the victory which could see him reclaim the world number one spot for the first time since March 2013.
The Open champion pulled his opening drive into the rough but had a gap through the trees and hit a superb low pitch to three feet, while Garcia had to settle for a par by two-putting from long range.
McIlroy then two-putted from long range on the par-five second for another birdie as Garcia missed from six feet, and there was already plenty of evidence that the rain-softened course was vulnerable to low scoring.
Five-time major winner Phil Mickelson had charged through the field with an eight-under-par 62 which featured 10 birdies and two bogeys, the left-hander having been disgusted with his opening three rounds.
"I don't know what to say, it kind of came out of nowhere," Mickelson said after finishing five under for the tournament. "The first two rounds really threw me for a loop. I have been struggling getting my short irons close and today they were right on.
"I made a lot of birdies and it was nice. Today was a big day to give me a lot of confidence and momentum heading into the US PGA next week."
England's Lee Westwood had also found some welcome form after four consecutive missed cuts, the 41-year-old carding a flawless 63 that did not go unnoticed by watching Ryder Cup captain Paul McGinley.
"If you look at the one pillar that has been consistent throughout the Ryder Cup success we have had since the 90s it has been Lee Westwood but you want to see Lee in form," McGinley said on Sky Sports 4.
"He is not going to get in on his reputation alone. He needs some form, he knows that and it's good to see him playing well today."
Westwood is currently outside the automatic Ryder Cup qualifying places.