But Koepka responded to the challenge, firing his tee shot to three-feet at the par-three 17th to regain the lead with a birdie, moments after his Japanese opponent had missed a six-footer.
Koepka then sank a six-foot birdie at the par-five 18th to edge Ikeda, who also birdied the last for a magnificent round of 10-under 61.
"To watch (Ikeda) fire 10-under was very impressive. I just hung in there and it was nice to finish with two birdies coming in," Koepka said after carding 65 to finish at 21-under-par 263.
It is the seventh career victory for 26-year-old Koepka, who has won once on the U.S. PGA Tour, once on the main European Tour and four times on the secondary European Challenge Tour.
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Koepka, ranked 21st in the world, finished second at Las Vegas two weeks ago.
(Reporting by Andrew Both; editing by Sudipto Ganguly)
