The 30-year-old, who had an injury-hit 2020 and did not win any events last year, won the Waste Management Phoenix Open earlier this month and continued that form with a six-under-par round of 66.

Koepka made seven birdies and dropped just one shot, finishing round two 11 strokes under par.

Three players are one shot back of four-time major champion Koepka - Australia's Cameron Smith and fellow Americans Collin Morikawa and Billy Horschel.

Morikawa played the round of day, a 64 which included birdies on each of the opening five odd-numbered holes.

Bryson DeChambeau, like Morikawa an American who won their first major last year, also shot 64, but is tied for 20th and only three under for the tournament.

Defending champion Patrick Reed of the United States is three shots off the lead in a tie for eighth, while compatriots Webb Simpson, Tony Finau and England's Matt Fitzpatrick are at nine under par.

Koepka has won just one WGC event before, the 2019 WGC-FedEx St. Jude Invitational, while the next six players - Horschel, Simpson, Morikawa, Smith, Finau and Fitzpatrick - have a combined zero WGC victories.

Northern Ireland's Rory McIlroy, aiming to complete a full set of four WGC titles, is six shots off the lead.

This tournament - moved from Mexico to Florida because of the coronavirus pandemic - is the first WGC event of the season.