A second-round 69 after his opening 71 left the 52-year-old at four under par at the halfway point at Augusta National.
Mickelson is a distant eight shots behind runaway leader Brooks Koepka, but only five back from second spot and brimming with belief.
"I'm going to go on a tear pretty soon," Mickelson said.
"You wouldn't think it. You look at the scores. But I've been playing exactly how I played yesterday, hitting the ball great, turning 65s, 66s into 77s. I'm ready to go on a tear."
Less than two years after Mickelson became the oldest major champion in golf history, the three-time Masters winner contending might have been less surprising in an alternate universe.
But 'Lefty' has been away from the public eye, skipping the 2022 Masters amid the controversy surrounding his inflammatory remarks about LIV Golf and the Saudi Arabia regime, then laboured on a leisurely tournament schedule on the breakaway tour.
The five-time major winner has only one top-10 finish and two top-20s in more than a year on the LIV circuit, where he's competing against just 47 other golfers at 54-hole, no-cut events.
His best finish in three LIV events this year is 27th. Nothing about his recent form indicated he would be a factor at Augusta.
Mickelson credited his Hy Flyers teammate at LIV, Brendan Steele, for helping him sort out recent driver troubles and said he was finally turning good ball-striking into good numbers.
As for whether that could happen as soon as this weekend, he said, "It's possible. Who knows when it will click. It could click tomorrow. I don't know.
"Part of it is just slowing my mind down and letting it happen and then it clicks. But that's kind of the biggest challenge in the game - not forcing it."
With Reuters
19°C