"It might be weeks, it might be months, it might be days, hours - I don't know," he said. "Last year I putted so well for a year and a half, and I've kind of overdone what I was doing, and I've got to kind of settle back in. [The grip] takes away some of the pressure."
Mickelson, whose sixth runner-up finish in the Open came last year at Merion Golf Club, hit 13 greens and carded three birdies and three bogeys. Two bogeys came in his last four holes.
"It's a good start; I didn't hurt myself any," he said. "I had a chance to get to 3-, 4-, 5-under today had I made some makeable opportunities. But I didn't throw anything away on some of the short ones."
Mickelson has been a focus of an FBI and Securities and Exchange Commission investigation into insider trading. The New York Times reported Thursday that federal authorities found no evidence of questionable trading of Clorox stock, but that he remains under investigation for trading involving Dean Foods.
"I'll continue to say - I haven't done anything wrong," he said after his round. "I'm willing to help out, love to help out any way on the investigation. So like I've said before, with an investigation going on, I'm not going to comment any further on it."
Rickie Fowler was just 10 years old when Payne Stewart won the U.S. Open at Pinehurst, but he called Stewart "one of my all-time favorite players."
So on Thursday, 15 years later, Fowler arrived for his first round of the Open wearing knickers and plaid socks in honor of Stewart, who died in a plane crash four months after his victory in the national championship.
"It was really special just because the fans were making comments out there, and it kind of made me remember and think about Payne and seeing the reactions from players walking into the locker room this morning," said Fowler, who opened with a 70. "Phil [Mickelson] is a good buddy of mine, and he gave me a smile, a thumbs-up."
For Fowler, the knickers were a one-time deal. "I think I'm back to pants" Friday, he said.
Justin Rose's opening round was similar to his first 18 last year at Merion Golf Club, where he was 3-over par after 13 holes but came back to finish 1-over.
Rose finished nine holes Thursday at 4-over par and again shaved 2 strokes off that score by the end, a 2-over 72. But he wasn't exactly thrilled with his short game, made evident by a chunked chip on his 17th hole that led to a bogey.
"My short game was very poor," he said. "I just have to work on that. You're going to have at least four or five [pitches and chips] a day. Get that sharp, and I feel confident for the rest of the week."
Rose called the scuffed chip "a hang-your-head moment, I suppose."