Hideki Matsuyama fell eight strokes off the pace of American Brooks Koepka after Saturday's third round of the Dunlop Phoenix.

Koepka, the defending champion and winner of this year's U.S. Open, blistered the par-71 Phoenix Country Club course, firing in eight birdies against a lone bogey to sit at 16-under par 197, four strokes ahead of South Korea's Lee Sang Hee.

After back-to-back 67 rounds on the Miyazaki seaside, Matsuyama managed just two birdies and two bogeys in an even-par third round and said he let his Japanese fans down.

"I wanted to improve my score, but I should apologize (to the gallery) for my boring golf," said Matsuyama, who has twice finished runner-up to Koepka in the States, at this year's U.S. Open and at the 2015 Waste Management Open.

Matsuyama missed a first-hole birdie putt and missed an eagle putt on the par-4 13th, when he settled for his second birdie and could not hide his frustration on the greens.

"They (my putts) weren't going in, so the stress was building."

"I don't think he (Koepka) is going to let up. If I play my best and get a miraculous score in the 50s, then perhaps I have a chance."