SHANGHAI (Reuters) - Unheralded Swede Rikard Karlberg shot a first round eight-under-par 64 to outshine a stellar field at the WGC-HSBC Champions on Thursday, finishing a shot ahead of Rickie Fowler at a wet Sheshan International Golf Club.

The 29-year-old, playing in the tournament for the first time, picked up shots at his first four holes and added five more birdies before finally marking a bogey on his card after finding a bunker with his drive at the 18th.

"It was just one of those days where it was a walk in the park. I committed to a lot of shots and hit them perfectly, just as I wanted," Karlberg said.

"I bombed my drives, I bombed my shots to the green and I stayed aggressive with my putts, even when the greens were getting a bit bumpy at the end of the day."

Fowler hit seven birdies in a flawless 65 to stand a stroke ahead of compatriot Daniel Berger, reigning champion Russell Knox of Scotland and in-form Japanese Hideki Matsuyama, who all carded 66.

"It's always nice to start with seven under par, I'm pleased with that," the American said.

"I knew with the soft conditions they were going to try to tuck some pins. It was going to be very scoreable if you drove it in the fairways and I was able to do that.

"I played very well tee-to-green and that made things a lot easier from there. It was a stress-free, simple round."

U.S. Open champion Dustin Johnson picked up a double bogey at the par four 16th to slump to a two-over 74, while the man immediately behind him in the rankings, world number three Rory McIlroy, dropped a shot at the last for a one-under 71.

Karlberg's better known compatriot, British Open champion Henrik Stenson, will start Friday's second round five shots off the pace after mixing five birdies and two bogeys in his 69.

(Reporting by Nick Mulvenney in Singapore; Editing by John O'Brien)