Cornish rookie Harry Hall took a three-shot lead halfway through the Charles Schwab Challenge, while he threw four birds in a row on the turn.
Hall followed the opening lap with 62 points to 66 and moved up to 12, by three margins. American Harris English lighting up his 66 round with a hole in eighth hole at Colonial Country Club Fort WorthTexas.
Argentina’s Emiliano Grillo is one more shot down after the best lap of the day with 65 points.
Former Walker Cup star Hall is on the hunt for his first PGA Tour win after finishing twice in the top 10 in his rookie season so far.
Starting at 10th, he bogeyed at 13th before reversing that shot two holes later and grabbing four straight birds around the turn to strengthen his position at the top of the leaderboard.
That run ended with a bogey on the third, but he smashed seventh and made his second attempt to escape a green bunker in eighth after tripping over the side.
The English fared better in eighth at 170 yards, where he produced his first ace in the hole since Jim Furyk in 2011.
Adam Schenk, Byeong Hun An and Robby Shelton finished fourth in the world, one shot behind Grillo and one shot ahead of Andrew Novak. Scottie SchefflerLosing in a playoff 12 months ago and getting their second straight 67 round.
Justin RoseThe winner at Colonial five years ago was ninth under 5 after a desperate 66.
Michael Block, 46, professional club from California who won an invitation after their 15th draw at the US PGA Championship, finished 15th above average on the 120-strong pitch but advanced by seven strokes in the opening 81 round. .