Oosthuizen birdied three of his first five holes and then after two early birdies on the back nine, carded an eagle at the par-5 15th en route to a flawless seven-under-par 64 at Club de Golf Chapultepec in Mexico City.

That left the South African World No.29 one shot clear of WGC debutants Briton Chris Paisley and Indian Shubhankar Sharma, and American Xander Schauffele who all shot 65s.

"I'm very confident with my driver at the moment. I've got a nice little cut going, and it's nice to be able to just aim left and swing away," Oosthuizen said.

The Australian contingent made a slow start, with Marc Leishman shooting a two-under-par 69 to be five off the pace.

Marc Leishman made a slow start in Mexico but is the highest-placed Australian, five shots off the lead. PHOTO: Gregory Shamus/Getty Images.

Leishman mixed his card with five birdies and three bogies to be in a tie for 13th.

Adam Bland was tied 23rd, a shot behind, while Brett Rumford (seven over) and Wade Ormsby (eight over) struggled.

World No.2 Jon Rahm, playing in high-profile group with World No.1 Dustin Johnson and Justin Thomas, birdied his final hole – the par-4 9th – to sit alone in seventh place, three shots off the pace after a 67.

Masters champion Sergio Garcia was in a share of eighth with four other players after a three-under 68 that included seven birdies and four bogies.

Thomas, fresh off his triumph at the Honda Classic, suffered a setback in his bid for back-to-back victories after mixing three bogies with two birdies for a one-over 72.

Defending champion Johnson, who came within inches of a hole-in-one at the par-3 17th, was in a share of 13th after six birdies, two bogies and a double bogie.