BOISE, July 26, 2013 (AFP) - Knox's 12-under par round at Hillcrest Country Club made the 28-year-old from Inverness the early clubhouse leader at 14-under 128 through 36 holes.
Knox began off the 10th tee with a birdie and followed with four pars in a row but added seven birdies and two eagles over the next 11 holes.
With a chance to finish even lower, Knox made pars on the eighth and nine holes to finish on 59.
It came just two weeks after American Will Wilcox shot 59 in the final round of a Web.com Tour event in Utah, which was the first 59 on the circuit since American Jason Gore accomplished the feat in 2005. Americans Notah Begay and Doug Dunakey fired 59s in the 1998 season.
Knox, who shared 45th at the US Open in his only major start this year, has played 10 US PGA Tour events this season and is in his ninth event on the developmental series this year.
His best PGA showing of the season was a share of 13th at last month's Travelers Championship the week after the US Open. His best career PGA finishes were shares of ninth last year at the Children's Miracle Network Classic and Frys.com Open.