TOWN AND COUNTRY, Mo. — Brooks Koepka is one of the longest hitters in golf, but most weeks he is a distracted driver, his focus fading in and out like a radio signal on a tree-lined mountain road. But four weeks a year, during the majors, Koepka has no problem locking in his concentration.

“I can really tune in in the majors, and I have no idea why,” said Koepka, who has won three of them and only one regular PGA Tour event. “They really get my attention.”

On Sunday at the 100th P.G.A. Championship, Koepka, 28, could easily have lost his focus. He was being chased by the two players he had idolized growing up: Tiger Woods, 42, and Adam Scott, the 38-year-old Australian.

A hard-charging Woods had the full-throated support of the fans crammed like packing peanuts into the 200-acre parcel of property that is Bellerive Country Club. Scott, whose swing Koepka loved as a child, joined him in the final twosome, dueling Koepka down the stretch as if paying homage to the P.G.A. Championship’s earlier incarnation as a match-play event.