SPRINGFIELD, N.J. — For Jimmy Walker, Sunday was his fourth successive day atop the leaderboard at the P.G.A. Championship. He had played throughout with an unruffled calm, a countenance that belied his inner unease about the outcome of the tournament.

Walker was a tenderfoot when it came to the crucible of the last hours of a major championship. He had missed the cut in his last two majors, never finished higher than tied for seventh at any major and was perhaps best known on the PGA Tour for his unusual hobby: astrophotography.

Now, in Sunday’s final round, he was being chased by battle-hardened golf heavyweights, like Jason Day, the world’s top-ranked player and the defending P.G.A. champ.

Undaunted, Walker had kept his competitors at arm’s length with a bogey-free round at the venerable Baltusrol Golf Club, and as he walked to the final tee, he held a three-stroke lead. Then Day, in second place, eagled the par-5 18th hole, and Walker suddenly had to par his final hole or face a playoff. Or worse, second place.