Rahm started the final round of the Sentry Tournament of Champions seven shots behind. He bogeyed his first hole. He was six shots back at the turn to Morikawa, who had yet to make a bogey the entire week at Kapalua.
"Bit of a crazy day, I'm not going to lie," Rahm said.
A wild hour featured a seven-shot swing in four holes when Rahm ran off three straight birdies and an eagle, and Morikawa, playing two groups behind him, made three straight bogeys on holes the rest of the field collectively played in 44-under par.
It ended with Rahm making one last birdie for a 10-under 63 and a two-shot win over Morikawa (72), who went from a sure victory to a footnote in PGA Tour history as the ninth player to lose a six-shot lead going into the final round.
"It's going to hurt, but I've got to get over it because we're still in the very early parts of the season," Morikawa said.
So began the bold new year on the PGA Tour of elevated tournaments that average $20 million in prize money in a bid to bring the best together more often and reward them amid the challenge of Saudi-funded LIV Golf.
Rahm is riding a big wave, winning for the third time in his past five starts worldwide.
"In my mind, I feel like since August I've been the best player in the world," he said.
For Morikawa, it was an epic wipeout even by Maui's standards.
He still was three shots ahead when he bladed a 25-yard bunker shot over the 14th green and made bogey. On the par-5 15th, his 5-wood leaked just enough right to go down the slope and leave his a daunting chip, up the hill with the strong grain of grass into him. He muffed the chip and took bogey. And then on the 16th, his wedge wasn't long enough and rolled off the false front of the green some 70 feet from the pin for a third straight bogey.
"He still shot 63," Morikawa said. "He definitely made the birdies when he needed to. But I also made bogeys. When you're getting bogeys at that time of the tournament they're costly. I definitely felt the weight of that."
He wound up tying a PGA Tour record for losing the largest 54-hole lead at six shots. Eight other players have done that, most recently Scottie Scheffler at the Tour Championship last year. That event has a staggered start to par based on FedEx Cup standings. For regular stroke play, the last occasion was Dustin Johnson in 2017 at the HSBC Champions in Shanghai.
He also became the first player in tour history to enter the final round with a lead of 6 or more strokes, shoot under par in the final round and fail to win the event, per ESPN Stats & Information research.
Morikawa looked to be in a state of shock as he walked down the 17th fairway, leading by as many as seven shots during the final round and suddenly finding himself two shots behind and running out of hope.
Rahm was surprised, too. He knew he was back in the game, and as he walked down the 17th fairway and looked at the video board, he saw his picture on the screen with a "1" next to his position -- not even a "T1" to indicate a tie.
Rahm said he looked over at caddie Adam Hayes and said, "What's going on?"
It was the second such collapse by Morikawa in a little more than a year. He closed out 2021 at the Hero World Challenge and had a five-shot lead with a chance to reach No. 1 in the world with a victory. He shot 76 and finished fifth.
Masters champion Scheffler had a chance to return to No. 1 this week if he finished in a two-way tie for third or better. He had to settle for a 70 and tied for seventh.
Rahm finished at 27-under 265, and it was a small measure of redemption. Last year he finished at 33-under par at Kapalua, which was a PGA Tour record that lasted only a few seconds. Cameron Smith finished at 34 under to win by one.
Rahm now is 60 under in his past two appearances at Kapalua. The victory was his ninth on the PGA Tour and 17th worldwide, and assures he will be back on Maui to start 2024.
He won $2.7 million from the $15 million purse at Kapalua, the first of the "elevated" events on the PGA Tour schedule. He also gets 25% of his Player Impact Program bonus money -- he finished No. 5 in the PIP for $6 million.
The Spaniard now has won in each of his seven full years on the PGA.
Tom Hoge had a 64 and tied for third with Max Homa (66). Hoge headed for the airport to go to Los Angeles to watch his school, TCU, play for the college football national championship against Georgia. Hoge then will go back to Hawai'i for the Sony Open.
Information from The Associated Press was used in this report.