Leishman needed birdies on Nos. 16, 17 and 18 to tie Jim Furyk's course-record 59. He made three pars.
"(Fifty-nine) entered my mind for a little bit," Leishman said. "And then I thought about (a tournament) in New Orleans a couple of years ago when I got greedy and ended up bogeying a par-5. You don't want to wreck a really good round."
Day threatened to do that by driving it into the right rough on No. 18. But he laid up and hit a nearly perfect approach from 87 yards, leading to a birdie.
Day and Mickelson are living parallel lives. Both are charismatic major winners, but neither has won a tournament in 2017. Both need to play well this week to secure a spot in the 30-man Tour Championship. (Day entered the week 28th in points; Mickelson is 36th.)
And most significantly, both recently split with caddies who were considered intrinsic to their games.
Mickelson is using brother Tim after his amicable divorce from Jim "Bones" Mackay. Day on Tuesday cut loose father figure Colin Swatton, saying: "Obviously this is a very hard time for both of us. My attitude coming to the course has not been good."
Mickelson had to be grumpy after missing the cut at the British Open and PGA Championship and shooting a final-round 75 in the first playoff event on Long Island.
He said he was in a "mental fog" until undergoing undisclosed "medical changes" before the PGA Tour's most recent event two weeks ago outside Boston. Mickelson tied for sixth and said his energy, focus and visualization skills have returned.
"Going bogey-free (Thursday) shows I'm much more in tune," he said. "It means I'm dialed into each shot."
He hit just eight of 14 fairways but 14 of 18 greens in regulation and putted solidly.
Day also hit eight fairways, averaging a massive 337 yards off the tee.
Day, who won the 2015 BMW Championship at Conway Farms, said of new caddie and longtime friend Luke Reardon: "He's one of my best mates, but I could completely hate him as a caddie. You don't know until you actually do it."
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