The youngest Australian in the field at the year's final major is also the leading Australian after the first round at Champions Country Club in Houston, Texas.

Ruffels once again showcased her steely nerve with a back-nine fightback to finish with an even-par round on the Cypress Creek layout, the more difficult of the two courses being used over the opening two rounds because of reduced daylight.

The 20-year-old sensation is just four shots behind American leader Amy Olsen.

"I was really happy with my round. I was just excited to get it going and can't believe the first round is already done," Ruffels said.

"Yeah, I was one over on the back nine, which is my front nine, so happy with the way I could bring it home."

A winner of the US Amateur crown last year and runner-up in her 2020 title defence, Ruffels said it was "really cool" to be in the top 25 and is confident of contending for the trophy as an amateur.

"It can be done obviously, so it's still early days," said the former Australian junior tennis champion.

"I was really happy with my round. I was just excited to get it going and can't believe the first round is already done." – Gabi Ruffels

"I mean, I'm not really a leaderboard watcher, but just trying to go out there, see how good I can do, and definitely happy with the round.

"Just keep going".

Hannah Green, the 2019 US PGA winner, is the next-best Australian, sitting at one-over par after a 71.

Minjee Lee was unable to take advantage of the easier scoring conditions on the Jackrabbit layout, a birdie at the last one of the few highlights in the World No.8's round of three-over 74.

The Australian No.1 has left herself playing catch-up at Cypress Creek for the remainder of her tournament.

Katherine Kirk was travelling nicely most of the day, even par through her first 13 holes, before collapsing down the stretch.

Kirk followed up a double-bogey six on the 14th with further dropped shots on the 15th and 18th to card a four-over 75.

Su Oh also came unstuck on the back nine at Cypress Creek.

After reaching the turn at one under following birdies on the 3rd and 6th holes, Oh racked up a horror triple-bogey seven on 11, then bogeyed 14 and 15 to also shoot 75.

Sarah Jane Smith mixed two birdies with five pars and a double in her round of 76 at Cypress Creek.

- Darren Walton, Australian Associated Press