Thomas is two-time defending champion.
A 64 has Cameron Smith on top at TPC Kuala Lumpur.
Apart from Thomas, eight other major winners are in the restricted field of 78 at the $7 million tournament, including Americans Keegan Bradley, Stewart Cink and Jason Dufner, and a trio of South Africans in Ernie Els, Louis Oosthuizen and Charl Schwartzel. Live coverage continues on Friday from 3.30am.
Last week: Brendan Steele won the Safeway Open.
Thomas has thrived under sweltering conditions in Kuala Lumpur in the last two editions to rank No. 4 in the world, but stuttered in his final nine holes after erring with two bogeys.
Death Toll Climbs to 23 as Winds Fan California Wildfires
As of October 11, there is no rain in the forecast for the next week and humidity levels are low. "This is just pure devastation". Pimlott said pilots couldn't see what was beyond their windows, much less what was on the ground.
So don't be surprised that the 23-year-old hasn't had a chance to readjust his goals after a whirlwind season - or that he's once again near the leaderboard after the opening round of the CIMB Classic. He will try to become the first player since Steve Stricker at the John Deere Classic (2009-11) to win the same PGA Tour event three straight years.
Xander Schauffele has only had two weeks off since winning the Tour Championship, and his Rookie of the Year trophy hasn't even had a chance to start collecting dust yet.
Smith, who broke his duck on the PGA Tour in May at the Zurich Classic of New Orleans alongside Jonas Blixt, made birdies at the third, fourth and fifth and picked up further shots at the seventh and ninth to approach the turn in 31.
Next week: CJ Cup at Nine Bridges.
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