KUALA LUMPUR (AFP) - The unheralded American Bronson Burgoon outshone Ryder Cup stars Justin Thomas and Paul Casey as he shot a nine-under 63 to lead the US$7 million (S$9.7 million) CIMB Classic on Thursday (Oct 11) - and then played down his round as "boring".

At the revamped TPC Kuala Lumpur, the 31-year-old chipped in for an eagle on the 18th hole, his ninth after starting from the 10th tee, and then added five birdies on his back nine.

That gave the world No. 162 a one-shot lead over compatriot Austin Cook, ranked 108th, on a low-scoring first day, with the red-hot Billy Horschel in a three-way tie for third at seven under.

Thomas, the two-time CIMB champion and former world No. 1, shot four birdies in his last five holes to move to three shots behind the leader, tied sixth at six under along with seven other players including England's Casey.

"I drove it well and I putted pretty good, so it was kind of boring golf, but that's always a good thing," said the Texan.

"I chipped in for eagle from the bunker on my ninth hole, which kind of boosted me, then I just kept going."

World No. 4 Thomas, winner of the event in 2015 and 2016, was happy with his start, especially the finish to his round.

"It was a good day. Kind of up and down. I got off to a good start and then kind of stalled out a little bit there in the middle," said the 25-year-old, who has eight PGA Tour wins in the last two seasons.

"I hung in there and just really didn't hit my irons very well. I just didn't take advantage of a couple of opportunities, but kind of found something there those last five holes and was able to at least make a good round of it today."

Horschel continued from where he left off in the FedEx Cup play-offs, where he finished inside the top-three in four starts and withdrew from the fourth.

Defending champion Pat Perez opened with a two-under 70 in the tournament jointly sanctioned by the PGA Tour and the Asian Tour.