In the opening round of the second of four PGA Tour playoff events, Mickelson played his first nine holes in 28. Despite a sensational shot that led to an eagle on his 11th hole, however, he had to be content with an 8-under 63.
The 18 putts that Mickelson holed at the TPC-Boston course totaled 134 feet. Earlier this year he had a legitimate chance at a 58 before finishing with a 60 in the opening round of the Phoenix Open, a tournament he won.
Kevin Stadler shot a 64 to finish the first round in second place while Hunter Mahan, Sergio Garcia and Roberto Castro were all at 65. The large group at 66 included Lee Westwood, Matt Kuchar, Steve Stricker and PGA Championship winner Jason Dufner.
Points leader Tiger Woods played without the sort of back discomfort he felt in the playoff opener last week in New Jersey and, while competing in the same pairing with Mickelson, shot a 68 that left him in a tie for 34th place. Adam Scott, who won the first playoff event, struggled to a 73 that put him in front of only six players.
The top 100 on the FedEx Cup points list make up the field this week and that group will be trimmed to 70 for the BMW Championship, which will be played in two weeks. Only the top 30 advance all the way to the Tour Championship, where the FedEx Cup champion and its accompanying $10 million will be awarded.
Mickelson played the back nine first on Friday and after making birdies on seven of those holes he suffered a bogey at the par-4 first.
He then bounced back at the par-5 second by hitting a 6-iron 213 yards over water to a dangerous hole location, a shot that finished within two feet of the cup. The short eagle putt got him back on track for a 59 or better, but he cooled off with five straight pars that wiped out those hopes.
After a sensational final-round 66 that brought him a three-shot victory at the British Open, Mickelson turned in a lackluster performance at the PGA Championship. His game showed signs of life last week, however, when he closed with a 65 to jump into contention at The Barclays.
A win this week would move Mickelson past from third to first on the points list.
Among those who had a big day Friday in a bid to stay in the playoffs were Stewart Cink and Ernie Els. Cink began the week in 85th place and his 66 put him in position to jump into the top 70. Els, who is 91st on the points list, also had a 66.