At 140 of the previous 144 British Opens, Mickelson would have been champion with his 17-under-par total at Royal Troon.
But after opening the tournament with the equal lowest score in major golf history, the American closed with an unblemished, career-best 65 featuring four birdies and an eagle – and still lost by three.
“It’s probably the best I’ve played and not won,” Mickelson said.
“That’s probably why it’s disappointing in that I don’t have a point where I can look back and say: `I should have done that or had I only done this’.
“I played a bogey-free round of 65 on the final round of a major. Usually that’s good enough to do it, and I got beat.
“I got beat by 10 birdies … I had to make 30-40 footers just to keep pace with him.”
In surpassing Arnold Palmer’s 10 second placings at the majors, Mickelson is now golf’s runner-up of runners-up behind only the great Jack Nicklaus, who fell similarly short on 19 occasions – as well as winning 18 times.
“I’m not sure how I’m going to feel,” Mickelson said.
“It’s disappointing to come in second, but I’m happy for Henrik. He’s really a great champion. We’ve been friends for some time.
“I’ve always thought that he is one of the best ball-strikers in the game and that major championships are perfectly suited for him.
“I knew that he would ultimately come through and win. I’m happy that he did. I’m disappointed that it was at my expense.”
Stenson’s victory denied Mickelson a sixth career major to match legends like Lee Trevino and Nick Faldo as well as a place in the history books as the oldest Open champion in 149 years.
Despite his graciousness in defeat, the 46-year-old admitted it was a tough pill to swallow.
“It’s not like I have decades left of opportunities to win majors, so each one means a lot to me,” Mickelson said.
The way Stenson and Mickelson battled it out and finished 11 and 14 shots clear of the field drew inevitable comparisons with the 1977 “Duel in the Sun” when Tom Watson edged out Nicklaus in similar fashion at Turnberry.
Mickelson admitted their battle down the stretch even felt like that to him too.
“I wanted to be more of Tom in that case than Jack.”
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