Teeing off in the final group of the final round Scott was always the best bet among the Down Under campaigners and halfway through the back nine he grabbed a share of the lead.
After a deflating bogey at the first it was an impressive fightback from the 2013 Masters champion who reeled off five birdies in a seven hole stretch from the 7th.
He walked to the 14th tee tied with Koepka but it was the American who played the better golf over the closing stages to claim the trophy.
“It was a bit of a slow start but everyone kind of struggled a little bit at the start it looked like,” Scott said of his opening.
“But then everyone got going around the turn, there were a lot of roars out there and it was good fun to be in it.
“But I just missed putts on 14 and 15 and Brooks made them and my momentum was gone and I didn’t play the last two holes very well.”
After a difficult year Scott said the finish was a positive but he was already looking ahead to the remainder of the PGA Tour season.
“I had high hopes for today, especially with four or five to play being tied with Brooks,” he said.
“But I’m definitely playing a lot better and moving in the right direction so that’s positive.
“It’s a good time of year with the playoffs coming up.”
Scott won’t be the only disappointed Australian leaving St Louis Sunday night, Jason Day also heading to the final round as a legitimate threat.
Despite not playing his best all week, Day was one of six players sitting four shots behind overnight leader Brooks Koepka and knew he needed something special Sunday if he was to contend.
He made an ideal start with a birdie at the second but on a Bellerive Country Club which had been giving up low scores all week, birdies were mandatory and pars meant you were going backwards and Day couldn’t find enough of the former.
A bogey at the 5th was offset by a stunning iron shot to kick-in range for birdie at the par-3 6th but that was as close as the 2015 champion would get.
He turned 1-under but gave that shot back at the 10th before five straight pars then a third bogey of the day at the 16th.
With those around him charging forward, the mistakes dropped him from T6 at the start of the day to T19 at the end, a result he will find less than satisfactory.
Ryan Fox was next best of the Australasians behind Day, the New Zealander posting his best finish in six major appearances with a final round 68 to be 6-under and T27.
It was his third 68 of the week and the 31-year-old continues to improve his play on the biggest stages.
Cameron Smith and Marc Leishman were the final two Australians to play the weekend, Smith producing one of the scores of the day Sunday to jump 21 places up the leaderboard to T56.
His final round 66 was bettered by only Tiger Woods, Tyrrell Hatton and Rafael Cabrera Bello who all signed for 6-under 64s but he will ultimately be disappointed with his week.
His Friday and Sunday scores of 66 were encouraging but surrounded by frustration with a Thursday 74 and Saturday 73 to never be a legitimate chance.
Leishman, too, will be less than satisfied with his week after a 2-over weekend saw him finish near the tail of the field.
The Victorian never found his rhythm in the Missouri heat and his opening 68 was his only sub 70 score of the week.
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