"I don't really want to tell you what I'm feeling right now", said McIlroy with only half a smile.
The in-form Rahm, who announced on Wednesday that he would be joining the European Tour in a bid to make the Ryder Cup team, also had five birdies in his confident opener, and Mickelson made it a four-way tie at the top when he emulated Fisher's birdie at the 18th.
Playing competitively for the first time since mid-January because of a rib injury, McIlroy fired a 3-under 68 in Round 1 to trail by just one shot.
Johnson birdied two of his last four holes to finish one under.
Westwood started at the 10th and suddenly raced into the lead with a superb mid-round birdie blitz, picking up six birdies in eight holes around the turn before giving one back when he three-putted from 60 feet at the fifth.
"The rib is fine", McIlroy said to Golf Channel.
"I've waited long enough to play", the world No. 3 said.
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McIlroy offered another apology on Tuesday and stressed his decision to play the round had nothing to do with politics.
"It's not as if I earn any more money as the world No.1, it's just nice to be able to say you're the best in the world at what you do".
"No, I didn't really sleep", McIlroy said.
"It's a great golf course, " Westwood said. But I haven't eaten so I'm a little weak The positive news is that I didn't feel the rib at all. "Even though most of those guys are my friends, it's tough to be sitting at home watching those guys doing what they're doing whenever you can't get out there".
The Holywood will be hoping to put the controversy behind him when he tees off at 5.35pm in the first round of the WGC Mexico Championship. He was three over par at the time.
After a solid west coast swing that showed Lefty still has a ways to go when it comes to finishing tournaments strong following two offseason sports hernia surgeries, he shot a 4-under 67 in Round 1 that included six birdies. To go there and see 30 secret service and 30 cops and snipers in the trees, it's just - I mean, it was just a surreal experience for me to see something like that. I'm glad that I only had three holes left after that, I was sort of lagging on the way in.
The four-time major victor then drove into a greenside bunker on the short par-four first hole and got up and down for birdie to reach two under par, while Johnson ran up a double bogey after hitting his tee shot out of bounds.