American Collin Morikawa emerged from a stacked leaderboard to win the 2020 US PGA Championship after a flawless final round in San Francisco.

At one point on the back nine, six players shared the lead - but an eagle on the 16th saw Morikawa, 23, go clear.

The world number 12, playing in just his second major, carded a six-under 64 at TPC Harding Park to reach 13 under.

And Koepka cranked up their friendly rivalry by belittling Johnson's solitary major win after day three, adding: "I like my chances.

Yet Koepka didn't factor in the leaderboard logjam as another American stole the show, one who is in just his second year on the PGA Tour.

Remarkably, Morikawa has more wins on the tour than missed cuts, and was second at the Charles Schwab Challenge in June before winning last month's Workday Charity Open.

"It's amazing," said Morikawa, who grew up in Los Angeles.

After a first round of 68, he said he felt more confident with caddie John McLaren back on the bag and having finished the third round two off the lead, there was genuine belief this could be his time.

Three birdies from his first 10 holes put Casey in contention to become England's first US PGA Championship winner since 1919 - and the oldest first-time major winner since 1967.

But he dropped a shot at the 13th after overcooking his approach, and despite responding with two more birdies, Casey graciously said Morikawa deserved victory after that "glorious" tee shot.