The pair leads by one stroke over seven players, including the 2018 runner-up, Akshay Bhatia, of Wake Forest, N.C., who at No.4 holds the highest position in the world ranking in the 156-player field. Reigning champion Michael Thorbjornsen, of Wellesley, Mass., is another stroke back at even-par 71, tied with seven other players as he seeks to join three-time champion Tiger Woods (1991-93), the only player to successfully defend his title in this championship.

The venerable club, which was founded in 1903, hosted the first of four US Opens on its Donald Ross-designed course in 1920, and this week is hosting its eighth USGA championship.

“Every fairway bunker I found, I was up against the lip. I took my medicine on those holes and trusted my wedge game.” – Karl Vilips

Vilips, 17, of Australia, who advanced to the Round of 32 in last year’s championship at Baltusrol Golf Club, recorded the lone bogey-free round of the day.

“You have to put it in the fairway off the tee,” noted Vilips, who is No.47 in the WAGR. “Every fairway bunker I found, I was up against the lip. I took my medicine on those holes and trusted my wedge game. Overall, I hit it well from tee to green and putted well.”

Mouw, 18, of Chino, Calif., also advanced to the Round of 32 at Baltusrol last year and reached the Round of 16 in the 2018 US Amateur at Pebble Beach. He captured the California State Amateur two weeks ago.

“I learned a lot last year playing in these tournaments,” said Mouw, who is No.26 in the WAGR. “I am much more mature. I like where my game is.”

Ricky Castillo, of Yorba Linda, Calif., who at No. 22 is the next-highest in the WAGR after Bhatia, also opened with a one-under 70.

- United States Golf Association