SHANGHAI (AFP) - US Open champion Brooks Koepka seized the first-round lead with a 64 at the WGC-HSBC Champions in Shanghai on Thursday as world No. 166 Gavin Green upstaged a host of bigger names.

The young Malaysian Green was joined in second, one shot behind pacesetter Koepka, by Kiradech Aphibarnrat after the Thai battled his emotions to perform one of the best rounds of his life.

World number one Dustin Johnson of the United States had a solid day at Sheshan International Golf Club and was tied 11th on four-under-par 68.

But 2016 champion Hideki Matsuyama had a nightmare opening to the defence of his crown, carding a forlorn 74 that left him struggling down in 67th.

Koepka, the American whose first major title came at Erin Hills in June, streaked to the top of the Shanghai leaderboard with his eight-under-par 64.

Green, 23, was a shock name up in second after going bogey-free.

He has never been higher than 157 in the world rankings and this was his first taste of a prestigious World Golf Championship (WGC).