Kuala Lumpur - Two-time defending champion Justin Thomas got off to a shaky start at the CIMB Classic on Thursday with a two-under-par 70, which left him languishing at the back of the pack.
Australian Cameron Smith, ranked number 151 in the world, took the lead with a sparkling eight-under-par 64.
Thomas was in tied 23rd place and other favourites also fared badly at the PGA Tour event in Malaysia, with both Japan's Hideki Matsuyama and India's Anirban Lahiri trailing the leaders.
Thomas is coming off the back of a breakthrough season during which he racked up five wins including a maiden major victory and was named the 2017 PGA Tour Player of the Year.
The world number four is hoping for a third consecutive win at the CIMB Classic, and opened his campaign with three birdies in the first nine holes at TPC Kuala Lumpur.
But the American, who started on the 10th, faltered after his promising start and hit three bogeys, leaving him to play catch-up against Smith.
It was his second-worst round in his CIMB Classic career.
Smith scored eight birdies to become the solo overnight leader.
"It was pretty simple really, hit a lot of good shots today and made a lot of good putts," he said.
"Same plan tomorrow. Really smart off the tee and just try and get in the fairway and then really aggressive into the greens and obviously hole some putts."
One shot behind Smith are Thailand's Poom Saksansin, American Keegan Bradley and freshly crowned Tour Championship winner Xander Schauffele, also from the US.
Schauffele, who hit a course low of 65, said he had "hit a lot of fairways and kind of made an easy day out of it".
India's Lahiri, in tied eighth place with a five-under 67, said he was "a little disappointed".
"Obviously got off to a red hot start and then couldn't capitalise on the (higher) scoring part of the golf course," he said.
The golfers could be in for a tough day Friday, with thunderstorms set to descend on Kuala Lumpur.
The $7 million CIMB Classic is jointly sanctioned by the Asian Tour and PGA Tour.
Leading scores from the first round of the CIMB Classic in Kuala Lumpur on Thursday (par 72):
64 - Cameron Smith (AUS)
65 - Poom Saksansin (THA), Xander Schauffele (USA), Keegan Bradley (USA)
66 - Kevin Na (USA), Gary Woodland (USA), Pat Perez (USA)
67 - Brendan Steele (USA), Anirban Lahiri (IND), Phachara Khongwatmai
(THA), Davis Love III (USA), Rafa Cabrera Bello (ESP), Kang Sung-Hoon
68 - Michael Kim (USA), Kim Wee (KOR), Hudson Swafford (USA), Thomas Pieters (BEL), Chez Reavie (USA)
69 - Jim Herman (USA), Ian Poulter (ENG), David Lipsky (USA), Jonas Blixt (SWE)
70 - C.T. Pan (TPE), Jazz Janewattananond (THA), Nick Taylor (CAN),
Ben Crane (USA), Morgan Hoffmann (USA), Jamie Lovemark (USA), Scott
Brown (USA), Justin Thomas (USA), Charl Schwartzel (RSA), Danny Chia
(MAS), Patrick Rodgers (USA), Luke List (USA), Hideki Matsuyama (JPN),
Kelly Kraft (USA)