Cornish rookie Harry Hall fired four consecutive birdies around the turn as he took a three-shot halfway lead in the Charles Schwab Challenge.
Hall followed his opening round 62 with a 66 as he moved to 12 under par, three clear of American Harris English who lit up his round of 66 with a hole in one at the eighth hole at the Colonial Country Club at Fort Worth, Texas.
Argentina's Emiliano Grillo is a shot further back after a best of the day round of 65.
Former Walker Cup star Hall is chasing his first PGA Tour win after chalking up two top 10 finishes in his rookie season to date.
Starting at the 10th, he bogeyed the 13th before getting that shot back two holes later and grabbing four straight birdies around the turn to strengthen his position at the top of the leaderboard.
That run was ended with a bogey at the third, but he birded the seventh and holed his second attempt to escape a greenside bunker at the eighth after plugging into the side.
English fared much better at the 170-yard eighth where he produced the first ace at the hole since Jim Furyk in 2011.
Adam Schenk, Byeong Hun An and Robby Shelton were tied for fourth, a stroke behind Grillo and one ahead of Andrew Novak and world number one Scottie Scheffler, who lost in a play-off 12 months ago and had his second consecutive round of 67.
Justin Rose, who won at Colonial five years ago, was tied for ninth at 5 under after a bogey-free 66.
Michael Block, the 46-year-old club pro from California who earned an invitation after tying for 15th at the US PGA Championship, finished last in the 120-strong field at 15 over par but improved on his opening round of 81 by seven strokes.