Teenage amateur Patrick Cantlay carded a course-record score of 60 to claim the lead at the Travelers Championship in Connecticut.
The 19-year-old enjoyed a near-perfect 10-under-par round, which included five birdies on the front nine before a brilliant eagle at the 13th.
The American's 60 is the lowest Tour round ever by an amateur.
He was the leading amateur at last week's US Open, finishing level par and sharing 21st place.
Despite being on course to become the first amateur to win a ranking event since Phil Mickelson clinched the 1991 Phoenix Open, Cantlay - a student at UCLA - has no immediate plans to turn professional.
"I'm not thinking about that right now," he said. "I'm going to try and take care of business this week and then see what's going on.
"But I'm going to stay amateur definitely for the Walker Cup, and my plans are to stay amateur until I finish college."
Cantlay leads by four strokes from compatriots Vaughn Taylor, who played 36 holes in the day after Thursday's play was interrupted by rain, DJ Trahan and Webb Simpson, as well as Brazilian Alexandre Rocha.