Yi, who shared the lead overnight with Steve Friesen, stands at 8-under- par 136 for a one-shot edge over three other players.
Brian Guetz, a winner at the Barton Creek Challenge last month, shares second place after a 3-under 69. Joining him at 7-under-par 137 were Stephen Gangluff (68) and Ron Harvey, Jr. (69).
Nine players are knotted two strokes back in fifth place, and 30 players overall are within five shots of Yi's lead.
Yi, a Texas native, began his round on the back nine and moved to 9 under with three consecutive birdies from the 13th hole. He reached as low as minus-10 with another birdie at the par-5 fourth.
After carding a bogey-free round Thursday, Yi stumbled to his first bogey of the tournament at the par-4 sixth, where he knocked his approach shot into a bunker.
Yi dropped another shot at the very next hole when he again failed to make par from a bunker.
'It was just a strange feeling making that bogey,' Yi said. 'It almost felt like a triple, but I was able to keep it together down the stretch.'
Yi was able to avoid another bogey at the ninth hole -- his last -- when he flew the green, but then chipped within 5 feet.
'That was a big finish for me and should help me going into Saturday,' Yi said. 'I'm looking forward to the next couple of days.'
Yi's best finish ever on the Canadian Tour was a tie for 10th place at the Corona Mazatlan Classic last year.
'I'm feeling really good about my game right now,' Yi said. 'Two bogeys in 36 holes is pretty darn good. Coming into the week, this is exactly where I wanted to be.'
The cut line fell at 1-over-par 145, with 71 players moving on to the weekend.
Among those making the cut was defending champion Stuart Anderson, who shot an even-par 72 on Friday to remain at four-under.
Friesen struggled to a 2-over 74 and is one of seven players tied with Anderson for 14th place.