EDMOND, Okla. (AP) — Darrell Kestner has withdrawn from the U.S. Senior Open after being treated for a heat-related issue during the opening round Thursday at Oak Tree National.

Kestner, 60, started at 2 p.m. from the 10th tee and completed 14 holes before withdrawing. Temperatures crept into the low 90s in the afternoon with high humidity. Golfers throughout the day said the heat was an issue.

Temperatures are expected to climb above 100 degrees before the end of the tournament.