Both players opened with par before Koepka struck first blood with a 10-foot birdie on the second to go one up.
DeChambeau missed a good opportunity to even the scores on the par-three third, misreading an 11-foot birdie putt to settle for a par, which was matched by Koepka.
After both men claimed par on the fourth, Koepka extended his lead on the par-five fifth with his second birdie of the day, while DeChambeau let a mighty drive go to waste by hitting a tree with his second shot and failing to get up-and-down.
Another misread putt from DeChambeau allowed Koepka to go three up on the sixth, which he added to with a fourth birdie on the par four eighth.
The event came to an abrupt end on the ninth when DeChambeau conceded after missing a long range putt to give Koepka a 4&3 victory.
“It was fun to come out here and settle this,” said Koepka. “It’s kind of like my major right now. I’m not going to lie. I just wanted to spank him.”
Additional reporting by PA Media.