Smith posted a birdie-filled two-under-par 68 to trail clubhouse leader Rory McIlroy by three shots after his first round at Southern Hills Country Club in Tulsa.

Australia's World No.4 leads the PGA Tour for birdies this season, with an average of 5.38 a round.

And he led the field on Thursday, too, with seven more birdies, with all coming from inside three metres.

"I really like the golf course. I hit my driver really good today, which freed me up into the greens, and was able to make a few birdies there on the back nine," Smith said.

"Just need to keep hitting fairways. My irons and putting feels really good, just keep those birdie looks coming."

Smith also missed three putts from between seven and 10 feet.

RIGHT: Rory McIlroy took the early lead after an opening round of 65. PHOTO: Ezra Shaw/Getty Images.

But his back-nine blitz of five birdies in seven holes at least saved The Players champion from shooting over par after he reached the turn at two-over.

Smith was in a tie for sixth when he completed his round.

McIlroy was alone on top at five-under, the former World No.1 and four-time major champion holding a one-shot lead over Smith's playing partner Will Zalatoris and fellow American Tom Hoge.

Matt Kuchar and Abraham Ancer were sharing fourth after firing three-under 67s.

Starting on the 10th, Smith could barely believe he didn't make a birdie start after his putt hung over the lip of the hole.

He waited as long as he could, but the ball wouldn't drop, and he had to settle for par before rebounding with a birdie on the 11th with a laser approach to the par-3.

But after missing the fairway right off the tee on 12, Smith went from rough to rough to card a dreaded double and give up two shots.

He birdied with the par-5 13th to return to even par only to drop successive shots on 16 and 17.

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Smith's brilliant fightback began with three consecutive birdies to open his inward nine and surge back into red figures.

After being forced to lay up on the par-5 5th after finding a fairway off the tee, Smith produced the shot of the day to set up his sixth and most unlikely birdie, almost holing out for eagle from 215 yards.

He picked up another shot on the 7th to climb to three-under, only to drop a shot on the brutal par-3 8th after finding a trap and being unable to get up-and-down.

Australia's two other morning starters – Cam Davis (72) and Matt Jones (73) – were seven and eight shots behind McIlroy, respectively.