RORY McILROY is poised to end Dustin Johnson's spell as world No 1 - after just two weeks.
A distinctly European flavour to the leaderboard is enhanced by Tyrrell Hatton, who is five under par and alongside Thomas Pieters.
Another Englishman, Andy Sullivan, is on five under after a 65.
Mickelson and Fisher both carded rounds-of-68 to find themselves seven-below par overall, while Thomas's 66 - despite three bogeys - leaves him in a chasing pack of three just off the leader.
Fortunately for Mickelson, his brother Tim had been alerted to the problem and was on hand to take over on the bag as his sibling looked to remain in contention for a third World Golf Championship title.
"I couldn't tell you any time I've ever searched myself on YouTube, but I clicked on it", said Spieth, who pulled up footage from the 2015 WGC-Bridgestone Invitational.
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"It was fun. It was the first time we were able to do that and it was fun". I have a whole new respect for those guys. "Bones is irreplaceable", he said.
"He's irreplaceable, he's one of the best in the business", Mickelson said of Mackay.
Playing in his first tournament since mid-January after being sidelined with a rib injury, McIlroy recorded an eagle for the second straight day to go with six birdies in moving to 9-under 133 through 36 holes. "But my brother's a great player, and he did a phenomenal job (Friday)". "But I felt like I could have been a few more ahead".
I would have taken a 65 starting today and I definitely would have taken a two-shot lead going into the weekend, so I'm in a great position.
And when he also bogeyed the third, the four-time major victor found himself a shot behind Johnson and Mickelson, who had chipped in for a birdie on the first and picked up another shot after chipping in on the fourth.
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