Johnson carded the equal best round of the day at the Glen AbbeY course in Ontario, a 7-under 65, to join overnight leader Kevin Tway and Korean pair Ben An and Whee Kim at the top of the leaderboard at 17-under.
With seven birdies and an eagle against two dropped shots Johnson moved from an overnight share of sixth to the top of the standings as he pursues a 19th PGA Tour victory.
The leading group are four shots ahead of their nearest challengers, Hudson Swafford and Rory Sabbatini, with six players at 12-under and sharing seventh.
Aaron Baddeley, who started the day on the same score as Johnson, struggled on Saturday and with a 3-over 75 plummeted 37 places down the standings to T43.
The day got off to a poor start with a 3-over front nine and treading water with two birdies and two bogeys on the inward half did nothing to advance his cause.
All three Australians playing the weekend were over par Saturday, Rod Pampling also shooting 75 to tumble 44 places to T57 and Cameron Percy signing for a 77 that dropped him 56 spots to T71.
There were no such issues for the leading groups, however, Kevin Tway’s 4-under 68 the worst of the top four on the day.
Sand, tree, wind ...July 28, 2018
An signed for a 66 while Kim was around in 67 to set up a thrilling showdown in Sunday’s final round.
Johnsons’ third round continued an impressive bounce back after missing the cut at last week’s Open Championship where he said he played good golf without it being shown on the scorecard.
“I felt like I was playing well going into the Open Championship and felt like I played well at the Open, I just scored really bad,” he said.
“I felt like my mental game wasn’t there. So I came here and practised a little bit and got my focus back right and have played a little better this week.”
While nobody went lower Saturday, Johnson will feel he let a few shots slip, especially after a bogey at the par-5 13th.
“I didn’t drive it quite as well as I’d like to today. It was a little bit windy today, so it made it a little bit tougher to hit fairways,” he said.
“But I felt that when I was in the fairway I hit good shots and made a lot of birdies.”
Johnson is in his comfort zone at Glen Abbey where he has twice been runner-up and was T8 in his only other appearance.
While he will be the man to beat Sunday Kevin Tway is also not short on motivation as he takes aim at a first Tour title at a tournament won by his father 15 years ago.
“Yeah, he did it in '03, actually, so that was 15 years ago,” Tway said of the possibility after his second round.
“It would be great to match with that. I talk to dad every day, and he's given me insight on this course. Although he won there (nearby Hamilton Golf Club), but he's played here many times.”
Tway will play in the second to last pairing Sunday alongside Whee Kim while Ben An will be with Johnson in the final group.
RULES - PRACTISING ON THE COURSE:
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