The Spaniard was starting the defence of his Race to Dubai title at Chapultepec Golf Club and he started the weekend ten shots off the lead at one under.
He had been four over after 27 holes but he came home in 30 in round two and brought that form into Moving Day in devastating fashion.
He was six under after seven holes and while a bogey on the eighth threatened to derail his progress, he made further gains on the tenth, 13th and 15th.
Better was to come on the 17th as he holed a gap wedge with one bounce for a hole-in-one that handed him a history making 61.
That was the course record at Chapultepec, the lowest round of Rahm's European Tour career and the joint lowest round in the history of the World Golf Championships.
"I've been hitting really good putts for the better part of a year but they just haven't gone in," he said. "Starting yesterday on my tenth hole they started going in and it carried on today.
"I basically had tap-in, tap-in, tap-in and ten feet for birdie and I thought: 'today could be a really special day'.
"Then backing it up with a great birdie on six and seven and a great tee shot on eight, which is a difficult tee shot, I thought it could have been the day it ended up being.
"I had a little bit of a hiccup, a three putt on eight and it's just a difficult hole, but everything balances out.
"I think a lot of us like to talk sometimes about how unlucky we get and I think today was one of those days where I got fortunate.
"I hit the right shots and I got the right bounces and I took advantage of it. There was a couple of holes where I could have gotten a little more unlucky
"I think we all compete and want to win tournaments when everybody is playing their best and if you win one of those stacked fields, it's a lot of fun and it's a great accomplishment, so hopefully I'm the one on top tomorrow afternoon."