Uresti matched Chris Wisler's 54-hole record from a year ago as he hunts down his first title on the Canadian Tour.
David Hearn also fired a 63 on Saturday and is tied with John Mallinger (64) and Stephen Gangluff (67) at minus-9.
There was a 30-minute weather delay after the final threesome hit their putts on 17. They returned and teed off on 18, but lightning returned and halted play for another 30 minutes.
All players finished their third rounds as some players completed their second rounds Saturday morning. Play was stopped for darkness on both Thursday and Friday, but the tournament is caught up and on schedule.
Uresti was 2 under at the turn, but started going really low. He eagled the par-5 12th, then rattled off three consecutive birdies from the 14th to take the lead.
Uresti makes his living on the PGA Tour, but was not in the field for this week's Bay Hill Invitational. He has made four of six cuts in 2005, but his best finish was a tie for 26th place at the Chrysler Classic of Tucson.
Hearn is also a member of the PGA Tour playing this week. He was 2 under on his round after the front nine and birdied the 10th, and like Uresti, eagled the 12th. Hearn birdied 15 and 17 to get within two of Uresti's lead.
'I needed a really good round to get back into this thing,' said Hearn, who missed the cut in his first three events on the PGA Tour this year. 'I've been hitting it well the past couple of days, but just couldn't make any putts. It feels good to get back in contention.'
Mallinger, a co-medalist at the California Qualifying School, was flawless on Saturday with four birdies and an eagle. He also collected his eagle at the 535-yard 12th.
Gangluff, a former member of the PGA Tour, mixed five birdies and two bogeys for his second 67 in as many rounds.
Stuart Anderson (65), Ben Bunny (68) and Brian Guetz (69) share fifth place at minus-8.
Derek Gillespie had an eventful first two rounds. He established a new course record with a 60 in the first round, then aced the eighth hole in his second round.
The third round was different for the 26-year-old. Gillespie struggled to a 3-over 75 and is tied for eighth with Doug LaBelle (66), Paul Danielson (66), Jason Schultz (66), Chris Baryla (69) and Rob McMillan (71). The group is knotted at 7-under-par 203.