TROON, 14 July 2016, (AFP) - The 2013 Open champion came within inches of becoming the first ever player to shoot 62 in a major championship when his birdie putt at the last lipped out, leaving him to settle for an eight-under-par round instead.
It was, though, only the ninth time a player has scored 63 at The Open and the first since Rory McIlroy, who did it in 2010 at St Andrews, which is a par-72.
Left-hander Mickelson's effort was also a new record on the par-71 course, beating the previous best of 64 set by Greg Norman in 1989 and Tiger Woods in 1997.
"It was one of the best rounds I've ever played and yet I want to shed a tear right now," said the 46-year-old American.
"To have that putt lip out, that's going to sting for a while."
He will take plenty of solace from the fact his round put him firmly in control at the end of the first round with a three-shot lead over nearest challengers Patrick Reed and Martin Kaymer.
On a day of glorious sunshine on Scotland's west coast, Mickelson's round was flawless, with no dropped shots and eight birdies.
That included at the par-five 16th, when he produced a superb shot out of a greenside bunker before holing for birdie, and then a two at the short 17th, where he said his four-iron from the tee was his "best shot of the day."
At a venue where the last six winners have been American, most recently Todd Hamilton in 2004, Mickelson must now manage the expectations on him ahead of the second round, when much tougher conditions, with rain and wind, are in store.
Reed, the 25-year-old Texan, had earlier laid down a marker with an impressive opening round of 66 that for several hours gave him the clubhouse advantage.
The highlight of his day was when he holed his approach shot for an eagle two at the par-four third. He also had five birdies and two bogeys.
The breeze blowing in from the Firth of Clyde was hardly nasty but it was significant enough to make life particularly difficult on the back nine for most players.
"It doesn't matter if the wind is blowing or not, that back nine is tough," said Reed.
German Kaymer, meanwhile, had five birdies and no dropped shots in his excellent 66.
Behind him are a host of players at four-under, spearheaded by the reigning champion Zach Johnson, who rather marred his day by bogeying the last two holes.
Also at four-under are the American quintet of Justin Thomas, Steve Stricker, Billy Horschel, Tony Finau and Keegan Bradley, Denmark's Soren Kjeldsen and England's Andy Sullivan.
As for the 'Big Four', it was a slightly underwhelming day.
World number one Jason Day admitted he had "a lot of work to do" after a two-over 73, while Jordan Spieth and US Open champion Dustin Johnson both had even-par 71s.
Rory McIlroy, the 2014 Open winner who missed last year's championship due to injury, was rather more upbeat after his two-under round of 69.
"It was good. I think if I would've stepped on the first tee and someone would have given me a 69, I probably would have taken it," McIlroy said after his round.
Elsewhere, South Africa's Louis Oosthuizen bagged the first hole-in-one of the week when his tee shot at the par-three 14th landed at the front of the green and dropped in.
The 2010 Open winner has recent form of such exploits having also managed a stunning ace at the Masters in Augusta in April.
"I thought it was just going to be right of it, and next thing I just saw it up against the flag and obviously the crowd going crazy," he said later.
Oosthuizen finished level par for the day after shooting a 71.
Other players found the going rather tougher, with 2001 Open champion David Duval of the United States shooting an 11-over-par round of 82.
He had reached the turn level par, but promptly bogeyed the 10th and then had a nine at the par-four 11th, which is only separated from the Glasgow to Ayr railway line by a four-foot high stone wall. Things never got better after that.
Scottish veteran Sandy Lyle, the 1985 Open champion, fared even worse, shooting an 85.
Collated first round scores in the the 145th British Open at Royal Troon Golf Club, Troon, Scotland on Thursday (GBR&IRL unless stated, par 71; a denotes amateur):
63 - Phil Mickelson (USA)
66 - Martin Kaymer (GER), Patrick Reed (USA)
67 - Soren Kjeldsen (DEN), Steve Stricker (USA), Zach Johnson (USA), Tony Finau (USA), Keegan Bradley (USA), Justin Thomas (USA), Andrew Sullivan, Billy Horschel (USA)
68 - Justin Rose, Shugo Imahira (JPN), Bill Haas (USA), Sergio Garcia (ESP), Yuta Ikeda (JPN), Rafael Cabrera-Bello (ESP), Thomas Pieters (BEL), Soomin Lee (KOR), Richard Sterne (RSA), Henrik Stenson (SWE)
69 - Andrew Johnston, Francesco Molinari (ITA), Adam Scott (AUS), Matt Jones (AUS), Gary Woodland (USA), Zander Lombard (RSA), Emiliano Grillo (ARG), Vijay Singh (FIJ), Rory McIlroy, Kodai Ichihara (JPN), Jamie Donaldson, Rickie Fowler (USA), Anirban Lahiri (IND)
70 - K.T. Kim (KOR), Alex Noren (SWE), Haydn Porteous (RSA), Byeong-Hun An (KOR), Matteo Manassero (ITA), Branden Grace (RSA), Bubba Watson (USA), Webb Simpson (USA), Kevin Na (USA), J.B. Holmes (USA), Justin Leonard (USA), Tyrrell Hatton, Jim Herman (USA), Kevin Kisner (USA), Ryan Moore (USA), Padraig Harrington
71 - Charley Hoffman (USA), Harold Varner III (USA), Ross Fisher, Danny Willett, Thongchai Jaidee (THA), Phachara Khongwatmai (THA), Robert Rock, Scott Hend (AUS), Mark O'Meara (USA), Ernie Els (RSA), Daniel Summerhays (USA), Lee Westwood, Jordan Spieth (USA), Jason Dufner (USA), Colin Montgomerie, Louis Oosthuizen (RSA), Matt Kuchar (USA), Matthew Southgate, Kevin Chappell (USA), Darren Clarke, Victor Dubuisson (FRA), Dustin Johnson (USA), Ryan Evans, Miguel Angel Jimenez (ESP)
72 - Hideki Matsuyama (JPN), Marco Dawson (USA), Ryan Palmer (USA), Nathan Holman (AUS), Jimmy Walker (USA), Marcus Fraser (AUS), Chris Kirk (USA), Charl Schwartzel (RSA), Nicolas Colsaerts (BEL), Greg Chalmers (AUS), Smylie Kaufman (USA), Scott Fernandez (ESP), Hideto Tanihara (JPN), Paul Lawrie, Thorbjorn Olesen (DEN), Rod Pampling (AUS), Dave Coupland, Russell Knox, Jordan Niebrugge (USA)
73 - Steven Alker (NZL), Brandon Stone (RSA), Paul Howard, Tommy Fleetwood, Russell Henley (USA), Jason Day (AUS), Richie Ramsay, Callum Shinkwin, David Lingmerth (SWE), Sang-hee Lee (KOR), Harris English (USA), Brendan Steele (USA), Mark Calcavecchia (USA), Luke Donald, Brandt Snedeker (USA), Matthew Fitzpatrick
74 - Colt Knost (USA), David Howell, Nick Cullen (AUS), Rikard Karlberg (SWE), Robert Streb (USA), Yosuke Tsukada (JPN), Jim Furyk (USA), Marc Leishman (AUS), Jon Rahm (ESP), James Hahn (USA), Bernd Wiesberger (AUT)
75 - Seung-Yul Noh (KOR), Graeme McDowell, Joost Luiten (NED), Clement Sordet (FRA), Todd Hamilton (USA), William McGirt (USA), George Coetzee (RSA), Jamie Lovemark (USA), John Daly (USA), Kiradech Aphibarnrat (THA), James Heath, Jeunghun Wang (KOR), Oskar Arvidsson (SWE)
76 - Anthony Wall, Satoshi Kodaira (JPN), Stefano Mazzoli (a) (ITA), Brian Gay (USA), Patton Kizzire (USA), Fabian Gomez (ARG), James Morrison
77 - Marc Warren, Kristoffer Broberg (SWE), Paul Dunne, Paul Casey, Ben Curtis (USA), Scott Piercy (USA), Yusaku Miyazato (JPN) 78 Danny Lee (NZL), Lasse Jensen (DEN), Shane Lowry, Scott Gregory (a)
79 - Jack Senior, Steven Bowditch (AUS)
82 - David Duval (USA)
85 - Sandy Lyle