TROON, 15 July 2016, (AFP) - The American had been in front overnight after equalling the lowest ever round in a major championship when he shot 63 in glorious conditions on Thursday afternoon.
He was back out early on Friday morning and this time had to deal with the wind and, above all, rain that left the players and the course on Scotland's west coast sodden.
But he still thrived, with four birdies, including at the short eighth, the Postage Stamp, when he almost holed his tee shot.
There were two bogeys on the back nine, but Mickelson leads by a shot going into the weekend from Sweden's Henrik Stenson.
"I thought it was a good round to back up the low round yesterday. I made one or two bad swings that led to bogeys but for the most part I kept the ball in play and played stress-free golf," said the American, who won the Open at Muirfield in 2013 and admitted he enjoyed the unpleasant conditions that can constitute a Scottish summer.
"I was actually more worried about yesterday's round than I was about these coming rounds because I feel very comfortable in the conditions to be able to shoot a number, shoot a good score."
He might not have said the same about the weather that swept in from the Firth of Clyde in the afternoon, however, and those teeing off after midday toiled as it got wetter and windier.
Stenson was also safely in the clubhouse by then and his stunning 65 was the low score of the day, allowing him to sit second on the leaderboard at nine-under.
The effort, which equalled the late Payne Stewart's record for the lowest second-round score at Troon, featured seven birdies and just one bogey and gave him every chance of finally winning at a major this weekend.
"I haven't been in contention for the last six majors and that was a big, big goal of mine to try and be up there and give myself a chance. So far, so good," said the 40-year-old, runner-up to Mickelson in 2013.
His fellow Scandinavian, Soren Kjeldsen of Denmark, is joint-third along with American Keegan Bradley on seven-under after both shot 68s.
Next is defending champion Zach Johnson, whose 70 left him at five-under and means he is still in the running to become the first player to win the Claret Jug in successive years since Padraig Harrington in 2007 and 2008.
All these players were out early on Friday, and it was no coincidence that only a small handful of players on the course in the afternoon avoided over-par rounds.
Among those were Rory McIlroy, who had four birdies and four bogeys in a 71 that kept him alongside the likes of Dustin Johnson at two-under, which could yet give him a chance of repeating his 2014 win at Hoylake.
Jason Day also survived the dreich Scottish summer weather, a run of four birdies in five holes on the front nine setting him up for a 70 that put him one-over for the championship.
"We totally expected to have conditions that were going to be difficult, but not to the point where it was blowing 30 to 40 miles an hour with winds and rain coming in sideways," said Australian Day, the world number one.
He lamented the luck of the draw that prevented anyone out later in the day from climbing the leaderboard, but at least he survived until the weekend.
Jordan Spieth also scraped through on the cut mark of four-over, along with the likes of Graeme McDowell and Masters champion Danny Willett.
But there were plenty of high profile casualties on a course that doesn't require the help of the elements to be unforgiving.
Former Open champion Ernie Els failed to make the weekend, while his fellow South African Louis Oosthuizen collapsed a day after recording a hole in one at the 14th.
The 2010 champion had a dismal 83, including a nine at the treacherous par-four 11th, and ended up 12-over.
Then there was Ben Curtis, the 2003 champion who was 18-over after an awful round of 83 that included an inexplicable 10 at the par-four third and seven bogeys, although there was a birdie at the last.
Collated second round scores in the British Open at Royal Troon Golf Club, Scotland (GBR&IRL unless stated, par 71): (a) denotes amateurs)
132 - Phil Mickelson (USA) 63-69
133 - Henrik Stenson (SWE) 68-65
135 - Keegan Bradley (USA) 67-68, Soren Kjeldsen (Den) 67-68
137 - Zach Johnson (USA) 67-70
138 - Andrew Johnston 69-69, Tony Finau (USA) 67-71, Charl Schwartzel (RSA) 72-66, Bill Haas (USA) 68-70, Sergio Garcia (ESP) 68-70
139 - Kevin Na (USA) 70-69, Martin Kaymer (GER) 66-73, Matt Kuchar (USA) 71-68, Rafael Cabrera-Bello (ESP) 68-71
140 - Dustin Johnson (USA) 71-69, Francesco Molinari (ITA) 69-71, Rory McIlroy 69-71, J.B. Holmes (USA) 70-70, Byeong-Hun An (KOR) 70-70, Jim Herman (USA) 70-70, Patrick Reed (USA) 66-74
141 - Emiliano Grillo (ARG) 69-72, K.T. Kim (KOR) 70-71, Tyrrell Hatton 70-71, Rickie Fowler (USA) 69-72, Anirban Lahiri (IND) 69-72
142 - Steve Stricker (USA) 67-75, Matt Jones (AUS) 69-73, Alex Noren (SWE) 70-72, Jason Dufner (USA) 71-71, Adam Scott (AUS) 69-73, Gary Woodland (USA) 69-73, Matthew Southgate 71-71, Webb Simpson (USA) 70-72, Yuta Ikeda (JPN) 68-74, Jamie Donaldson 69-73, Kevin Kisner (USA) 70-72, Russell Knox 72-70, Richard Sterne (RSA) 68-74, Padraig Harrington 70-72
143 - Jason Day (AUS) 73-70, Harold Varner III (USA) 71-72, Mark O'Meara (USA) 71-72, Andrew Sullivan 67-76, Marc Leishman (AUS) 74-69, Miguel Angel Jimenez (ESP) 71-72, Greg Chalmers (AUS) 72-71, Darren Clarke 71-72, Ryan Moore (USA) 70-73
144 Charley Hoffman (USA) 71-73, David Howell 74-70, Branden Grace (RSA) 70-74, Justin Thomas (USA) 67-77, Scott Hend (AUS) 71-73, Daniel Summerhays (USA) 71-73, Lee Westwood 71-73, Thomas Pieters (BEL) 68-76
145 - Marco Dawson (USA) 72-73, Ryan Palmer (USA) 72-73, Nicolas Colsaerts (BEL) 72-73, Zander Lombard (RSA) 69-76, Thongchai Jaidee (THA) 71-74, Justin Rose 68-77, Jon Rahm (ESP) 74-71, Luke Donald 73-72, Soomin Lee (KOR) 68-77
146 - Graeme McDowell 75-71, Haydn Porteous (RSA) 70-76, Colin Montgomerie 71-75, Bubba Watson (USA) 70-76, Danny Willett 71-75, Kevin Chappell (USA) 71-75, Harris English (USA) 73-73, Jim Furyk (USA) 74-72, Kodai Ichihara (JPN) 69-77, Ryan Evans 71-75, Paul Lawrie 72-74, Patton Kizzire (USA) 76-70, James Hahn (USA) 74-72, Brandt Snedeker (USA) 73-73, Jordan Spieth (USA) 71-75