Spieth arrives at Bellerive Country Club in St. Louis needing a victory to become only the sixth player to win all four modern majors – joining Gene Sarazen, Ben Hogan, Gary Player, Jack Nicklaus and Tiger Woods.

The 25-year-old Spieth has made a habit of etching his name next to the giants of the game, and his path to golf's elite club has been all kinds of spectacular.

Last month's Open Championship at Carnoustie provided the latest glimpse into Spieth's brilliance and flaws.

Spieth during a practice round prior to the 2018 US PGA Championship. PHOTO: Andy Lyons/Getty Images.

He charged to the top and held a share of the lead entering the final round, only to disintegrate into the Scottish links with a five-over-par 76 that dropped him well out of contention.

Somehow, though, the experience left Spieth more inspired than discouraged.

"I believe that my best golf has yet to come, absolutely," he said.

"Even better than (2015), all that, yeah. It's all there, and it's moving in the right direction. So I'm actually very pleased coming out of this week."

"I believe that my best golf has yet to come.” – Jordan Spieth

To anyone other than Spieth himself, he seems a great distance from his hallmark 2015 form when he captured the Masters and US Open, finished tied for fourth at the Open Championship and was runner-up at the PGA.

Last week's outing at the WGC Bridgestone Invitational saw Spieth finish tied for 60th and added to a season of inconsistency.

It has now been more than a year since he recorded a PGA Tour victory.

The 2017 Open Championship at Royal Birkdale was Spieth's last triumph and a keen reminder of his mercurial golf, after he squandered a three-shot lead in four holes only to channel his most sublime play to seize the Claret Jug.

The 2016 Masters is the flipside for Spieth – he let slip a five-shot lead on the back nine during a collapse that echoed far beyond the reach of Augusta National.