McIlroy was 2-up with three to play in his semi-final against Cameron Young before losing on the first extra hole, while Sam Burns beat defending champion and World No.1 Scottie Scheffler on the 21st hole at Austin Country Club.

American Burns went on to thrash his 18th-ranked compatriot Young 6&5 in the final to claim his fifth PGA Tour title and the first prize of $3.5 million ($A5.3m).

"What a week. I'm so tired," World No.15 Burns told NBC.

"I felt like I found something towards the end of my match with Scottie and was able to carry that on this afternoon."

Young drew first blood in the final with a birdie on the 2nd, but Burns hit back in style with eight birdies in the space of 10 holes to complete a comprehensive win.

The one-sided contest was in stark contrast to the morning's semi-finals, which both went to extra holes and produced shock results.

"What a week. I'm so tired. I felt like I found something towards the end of my match with Scottie and was able to carry that on this afternoon." - Sam Burns.

McIlroy fell behind against Young after making a bogey on the 3rd, but bounced back to birdie the 5th, 6th and 9th for a two-hole lead at the turn.

Both players birdied 11 before McIlroy hooked his second shot on the par-5 12th from a fairway bunker into the water to see his lead halved.

However, an aggressive tee shot on the short 13th set McIlroy up for a birdie and he remained 2-up until Young birdied the par-5 16th.

Young also birdied the 18th and the 25-year-old then birdied the first extra hole, the par-5 12th.

"Feeling obviously pretty bad, two up with three to play, but Cam played great," McIlroy said.

"I just didn't do enough over those last four holes to close him out and when you don't do that and you're up against a player of Cam's calibre that's what's going to happen."

In the first semi-final, Burns made a flying start with a hat-trick of birdies to open up a three-hole lead, but bogeyed the 5th and Scheffler then birdied the 6th and 8th to get back on level terms.

Further birdies on the 9th and 10th, as Burns bogeyed both holes, gave Scheffler a two-hole lead before Burns birdied the 13th and 15th to get back to all square.

Burns almost holed his tee shot on the 17th to edge in front but Scheffler birdied the 18th to keep the match alive and both players birdied the first extra hole.

Scheffler missed from three feet on the 13th to win and Burns took full advantage, holing from 15 feet on the 14th to seal a dramatic victory.

"I played pretty solid most of the day," Scheffler said.

"I hit a bunch of lips today. Especially towards the end of the round I hit a lot of putts that I thought were going in that didn't.

"It's a bit frustrating ... I almost went four down through four and put up a good fight and Sam played really well."

In the third place play-off McIlroy got the better of Scheffler 2&1 having led for the entire match that made it all the way to the 17th hole.