'Unbelievable': US young gun lassos LIV Golf's finale

Australia's Cameron Smith has blown his shot at snapping a three-year winless run as American tyro Michael La Sasso upstaged two superstars to snatch LIV Golf's season-ending title in Indianapolis.

After starting the final round just one stroke off the lead, Smith's Sunday challenge all but a evaporated with a dispiriting and uncharacteristic three-putt bogey on the very first hole.

Five dreary hours later Smith signed for a lacklustre two-over-par 72 to slump to a share of 17th at 10 under for the championship.

That honour belonged to Marc Leishman, who fired a final-round 67 to finish joint 10th at 12 under, with Lucas Herbert's sizzling 64 vaulting him to equal 17th with Smith.

As Smith collapsed, multiple-major-winning golf titans Jon Rahm and Bryson DeChambeau emerged as third-round leader La Sasso's biggest threats.

At one point, Rahm and DeChambeau looked to be duelling for the title as La Sasso slipped two shots behind.

But the 22-year-old rallied to snare the trophy with a birdie at the par-5 18th in a steely closing round of 65 to prevail at 18 under.

Rahm opened the door for La Sasso with a bogey on his last hole of the day, the par-4 first, in an otherwise dazzling, birdie-filled final-round 63.

"I don't even know what to say," La Sasso said after collecting the $US2 million winner's cheque.

"I thought I played great all day and just waited for my moment and it finally came at the last."It's unbelievable. I'm at a loss for words."