Brennan Webb carded a 5-under 67 and is alone in third place at 10-under-par 206.
Johnson mixed four birdies and two bogeys on his front nine to make the turn at 2-under 34. Johnson added birdies at 12 and 14, but those birdies only tied him for the lead with Seki, who was already in the clubhouse.
Johnson elected to lay up at the par-5 16th, and the decision worked well. He knocked his third shot to 4 feet and converted the birdie putt for a one-shot lead. At the 17th, Johnson sank a clutch 6-footer to save par.
He nearly holed an eagle putt at the 18th, but had to settle for birdie.
'I am a good wedge player and I am going to play to my strengths,' said Johnson. 'If you can go out there and play the rest of the course the way the conditions dictate, you should be able to take advantage of the par-5 holes. If you can do that, you are going to have a decent score.'
Johnson will be in search of his fourth win on the Canadian Tour. This would be the first victory where he was alone in the lead with one round to go. His first victory was a weather-shortened event, for his second he shared the third-round lead and his most recent win he came from one stroke back.
'If I can just keep doing the things I've been doing, someone is going to have to take it pretty low to beat me,' said Johnson.'But I was thinking about it on 18 today. This is why I play this game. Having this feeling, with all the pressure, the tournament on the line, that is why most of us play.'
Blaine McCallister (66) and Scott Gibson (68) share fourth place at 9-under-par 207. John Humphries (67), Tom Johnson (69) and Michael Harris (69) are tied for sixth at minus-8.
Chris Parra (66), Brien Davis (67), Brian Nosler (69), Scott Taylor (70) and Matt Bettencourt (71) are knotted in ninth place at 7-under-par 209.