Jeff Ritter, digital development editor, GOLF.com: Stenson has never had a 54-hole lead in a major and never played in a final group.
But like boxers landing scoring punches they traded the lead again until by 18 the robot had fully rebooted to sit atop the leaderboard. It was less a conclusion of their head-to-head pairing than an adjournment.
Mickelson gave up the lead he held after the first two rounds.
Twice major victor Jordan Spieth suggested the critics were being "unfair" to him and urged everyone to be less negative about his performances after returning a one-over 72 in the British Open third round on Saturday. But to do so, he will need to outlast an all-time great who is putting on a vintage performance.
"I don't necessarily feel that much pressure", he said.
Mickelson, 46, is looking to become the oldest player to win the Claret Jug since 1867 and the fourth-oldest major victor ever. Mickelson is known to make mistakes when faced in difficulties but he also learned from these and won already five majors. What he won't have is the benefit of the lead. Phil Mickelson and Henrik Stenson will do it all over again at the bottom of the tee sheet in the final round. She missed a short birdie putt on the final hole. "Maybe it would have given me a little bit more of a cushion going into the back nine".
Phil Mickelson and Henrik Stenson are hardly enemies, but even friends would be getting sick of each other by now with the time the two have spent together this year. Moments before the start of his round Saturday, he stood behind the first tee box, looked around and smiled for several seconds.
Dustin Johnson appeared ready to challenge Stenson and Mickelson - until he took a triple-bogey seven on the par-4 11th hole. His lip-out for 62 on Thursday - which would have set the lowest score for a single round in major-championship history - will live on forever in highlights, a painful reminder of how cruel the game can be.
Playing his second to the par-five 16th, the Northern Irishman was unhappy with his shot and immediately launched the club forward, resulting in the head coming away from the shaft as it bounced down the fairway.
The two were separated by more than one shot only once all day, when Mickelson holed a 20-foot birdie at the 13th.
"I'll get it reshafted tonight and all will be well in the morning", said the Northern Irishman at the end of his two-over-par 73.
"I got lucky the ball didn't go in the gorse", he said. I found a way to kind of settle in and hit some shots and then find ways to make pars on the times that I hit some poor shots.
Phil Mickelson of the United States look down the line of his putt on the 4th green during the third round of the British Open Golf Championship at the Royal Troon Golf Club in Troon, Scotland, Saturday, July 16, 2016.
The two exchanged blows all round, flipping back and forth at the top of the leaderboard before Stenson used the two back nine par-3s to get ahead.
Stenson, bidding to become the first Scandinavian male to land a major, made his intentions clear when he birdied the opening hole.
It's proof of how impressively Mickelson and Stenson have played. Haas shot 69, while England's Andrew Johnston had a 70 and is fourth at five under par.
A sixth major title is still within grasp of the People's Champion. It will feature an American versus a European, like a Ryder Cup without the teams.
"There's only one thing that matters tomorrow", Stenson said.
Josh Berhow, staff producer, GOLF.com: Part of me wants to say neither, but the rest of the field is just so far back it will be tough for them to catch up.