South Africa's Louis Oosthuizen carded a seven-under-par round to lead after day one at the World Golf Championships (WGC) event in Mexico.
"It's probably the worst I've ever felt over the ball in my life".
Johnson added: "Coming in here past year, I think I felt like everything was really, really, really good in the golf game".
"I want to stay number one but I want to win every golf tournament I play in also so if someone plays well and beats me and gets to number one, they deserve it", the 33-year-old American said.
The Englishman birdied the 10th and briefly topped the leaderboard after holing from 40 feet at the 14th and picking up another shot at the par-five next, only to miss a seven-footer to save par at the 16th. "Up to previous year, I was watching these players on TV and now playing here this week is a dream come true".
To be fair, Oosthuizen is hardly a journeyman and he's certainly comfortable on the biggest stages (see Open Championship, 2010); and Rahm, who finished third here past year in his debut and can overtake Johnson this week as world No. 1 depending on a variety of scenarios, has emerged as a bona fide world-beater.More news: Kushner group got millions in loans after a White House meeting
"Had to put in a lot of hard work", he said. He was slowed significantly by mononucleosis in December, restricting his preparations for a new year to just four hours of practice.
These days, he does employ the grip it-and-rip it approach all the time, which previously earned him the nickname "Asia's John Daly". "That's where I was aiming", Thomas said.
"I was super impressed how nice he was", she said. "I like the course, it kind of suits my strengths".
How can you watch the WGC in Mexico? The other six are Gavin Green (Malaysia), Li Haotong (China), Shubhankar Sharma (India) and Japan's Yuta Ikeda, Satoshi Kodaira and Yusaku Miyazato.
World No. 1 Dustin Johnson came to the Club de Golf Chapultepec as the betting favorite to win his sixth career World Golf Championship title - and he very almost carded an ace on his eighth hole of the day.