In a text message to some national golf writers, Mickelson said he was "embarrassed" by his action of hitting a moving golf ball in Saturday's third round, a move that almost got him disqualified.
The last back-to-back winner was happy to watch the next generation achieve the rare feat, and couldn't wait to celebrate it.
The two-time U.S. Open Champion anticipated greatness - and wanted to go pro at age 16.
Koepka, who didn't touch a golf club for three months after suffering a wrist injury last November, became the seventh player in history to go back-to-back.
The European Tour star made an early run with a championship record-tying 63 but lamented a missed 8-foot birdie putt on 18. He lost to Brooks Koepka by one.
The 28-year-old co-leader after three rounds, Koepka will attempt to become the first player to win back-to-back Opens since Curtis Strange in 1988 and 1989,
Dustin Johnson shot a 77 but still managed to be part of a four-way tie for the lead on a day when USGA officials admitted the greens were too severe late in the day.
Lefty got away with a blatant, premeditated violation, but the darling of the U.S Open and the New York region remains in the field.
Mickelson, the winner of five majors and one of the most popular players of his era, received a 2-stroke penalty. "I don't mean it disrespectfully," he said.
Mickelson is 10 shots behind 36-hole leader Dustin Johnson but he's not giving up yet in his attempt to win the Open for the first time.
The 2016 Open champion seems ready to win another one thanks to a solid game and the mental makeup needed to deal with the challenges of a major.
The most popular golfer in the world finished 10-ver and missed the cut at the U.S. Open, but he finished with a flourish.
The BMW Philadelphia Amateur will conclude Saturday with a 36-hole championship match between Huntingdon Valley's Andrew Mason and Manasquan River's Jeremy Wall.