For all the times Mickelson has contended for that elusive major, he has never gone into the final round with no one in front of him. Better yet, the last 13 champions at Augusta National have come from the final group.
And Tiger Woods is nowhere to be found.
'I did want to be in the final group,' Mickelson said. 'I did want to be in a position where I didn't have to change my game plan to try to catch anyone. That's the nice thing about being in the lead right now.'
Still, this Masters is far from over.
DiMarco is just as hungry for his first major, and he was equally flawless on a tough course that gobbled up 36-hole leader Justin Rose and everyone around him.
DiMarco shot a 4-under 68 and joined Mickelson at 6-under 210.
'He's going to have a lot of pressure on him, too, because he's going to try to get that monkey off there,' DiMarco said, referring to Mickelson's 0-for-42 mark in the majors. 'It's going to be fun.'
An Englishman has a chance, too, although it isn't Rose.
Rose stumbled to an 81, the worst score by a 36-hole leader at the Masters since Lee Trevino shot the same score in 1989, and wound up nine shots behind.
Paul Casey picked up the slack with a 68 and was two shots out of the lead.
Experience is right behind them.
Ernie Els, a three-time major winner who craves a green jacket, recovered from a near disaster on the 11th hole to shoot 71. He was at 213, along with two-time Masters champion Bernhard Langer (69) and K.J. Choi (72).
'I've been in this position quite a few times, but I've been chasing Tiger,' Els said.
That won't be the case Sunday. Woods fell behind from the start, then tumbled out of contention with a double bogey on the par-5 13th. He wound up with a 75 and was nine shots out of the lead.
'I put myself pretty far back going into tomorrow,' Woods said, although he refused to count himself out. 'If I can get it to even par or under par going into the back nine, I'm right in the ball game. Because as we know, anything can happen on the back nine here.'
The attention will be on Mickelson, a supremely talented player with 22 victories on the PGA Tour. Only two other players - Harry 'Lighthorse' Cooper and MacDonald Smith - have won more often without capturing the four tournaments that matter the most.
Mickelson was close at Pinehurst five years ago in the U.S. Open until Payne Stewart made a 15-foot par putt on the final hole to beat him by one. He was close in Atlanta at the 2001 PGA Championship until David Toms beat him with a par on the final hole.
This will be the seventh time he goes into the final round at a major within two shots of the lead.
But this looks like a new Mickelson. He is in control off the tee, playing from the short grass instead of the trees. Even more impressive is that he has played his last 32 holes without a bogey.
'It's a much easier game when you keep it in play,' Mickelson said. 'I wish someone had told me this earlier.'
DiMarco has some Masters experience to draw from. He was the 36-hole leader three years ago, and kept his composure playing in the final pairing with Woods, shooting 72. DiMarco wound up 10th that year, but learned from it.
'That springboarded me to know I can do bigger and better things,' he said.
Even though Rose had a two-shot lead and had played with great confidence over two days, bright sunshine and a crisp breeze made this a day ripe with possibilities.
It didn't take long for it to unfold.
Rose hit is opening tee shot into the bunker, the start of a nightmarish nine holes. He flew over the green and made bogey, and his lead was gone one hole later when his wedge again was too strong and landed in the front row of the gallery for another bogey.
The scorecard for the front nine - six bogeys, three pars, one stunning collapse.
Mickelson and DiMarco wasted no time taking advantage.
DiMarco made the only birdie on the round on the par-3 fourth and also picked up birdies on the par 5s for a share of the lead.
Mickelson electrified the gallery with a collection of smart shots and key putts. Cheers burst from the third green when Lefty drained a 20-foot birdie, and they were even louder when his 25-foot dropped for a par on No. 6 that kept him in a share of the lead with DiMarco and Els.
More than that, it seemed to give Mickelson a surge of confidence. He stuffed a wedge into 2 feet for birdie on the seventh. Then Mickelson showed why he was golf's best wedge player by following that with a pitch on the par-5 eighth that skipped over the ridge and slid slowly past the hole for another easy birdie.
Els was poised to stay with him, but a solid round suddenly veered way off path.
He hooked his tee shot so far left into the trees at No. 11 that he was lucky to have a shot out to the fairway. Els had to remove one large branch, then seek three rulings - all the way to rules chairman Will Nicholson - to be able to get a clean lie.
Els was happy to escape with bogey, no harm there since No. 11 is the toughest hole on the course. The easiest hole turned into a headache. From great position in the fairway, Els hit into the front left bunker on the par-5 13th, from where it was nearly impossible to keep it on the top shelf. It rolled down the ridge to 45 feet, and he compounded the blown opportunity with a three-putt bogey.
Even so, Els is only three shots out of the lead and trying to catch a couple of guys trying to win their first major.
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