The 25-year-old from Dumfries stormed into contention in one of the second-tier circuit’s majors on the back of an eight-under-par 64 at Zhailjau Golf Resort in Almaty.
On 19-under-par, he has the leader, American Sean Crocker, in his sights in the battle for a top prize worth €72,000.
Johnston carded an eagle and seven birdies, five of which came in the final seven holes as he came home in 31.
The Scot, who is in his first full season as a professional, won the Andalucia - Costa del Sol Match Play earlier in the year but has since slipped to 40th in the Road to Ras Al Khaimah Rankings.
However, he has set up a chance to secure a step up to the European Tour next season as a win or even a second-place finish would effectively ensure a top-15 finish on the money-list.
Crocker, a 22-year-old who was born in Zimbabwe but now lives in Florida, set a new course record with a 10-under 62 as he moved to 20-under for the event.
This is just his seventh Challenge Tour event but he gave a glimpse of his potential when fnishing joint-third in Euram Bank Open in Austria in July, the week after he’d made the cut in The Open at Carnoustie.
Calum Hill (71) and Ewen Ferguson (68) are sitting joint-11th on 12-under, one ahead of David Law (71) as he bids to cement his place in the top 10 on the money-list.
On the European Tour, David Drysdale recovered brilliantly from a scrappy front nine to stay in the hunt heading into the final round of the KLM Open in the Netherlands.
Lying joint-third overnight on the back of a 65 - his lowest score of the season - in Friday’s second round, the 43-year-old was out in 36, one-over par, at The Dutch after dropping shots at the eighth and ninth.
Digging deep, though, in a bid to stay in the mix, he birdie the 11th, 13th and 14th to come home in 33 and a 69.
That left him sitting joint-sixth on 10-under, two shots behind Chris Wood after the lanky Englishman moved into pole position with a 65 that contained seven birdies.
The 2016 Ryder Cup player leads by a shot from China’ Ashun Wu, who slipped out of top spot after a 71, as well as Japan’s Hideto Tanihara (63) and Wood’s compatriot Jonathan ‘Jigger’ Thomson.
Connor Syme (72) and Scott Jamieson (70) are the next best among the Scots to make the cut, sitting in a share of 43rd spot on two-under.
Bradley Neil slipped 48 places to joint-57th following a disappointing 76 while Richie Ramsay is sharing 67th place after a 74.
Elsewhere, American Amy Olson carded a third-round 65 to open up a two-shot lead after the third round of the Evian Championship, the final women’s major of the season, in France.
She carded an eagle and four birdies in a bogey-free effort to head Korea’s Sei Young Kim (64) with Olson’s fellow American, Mo Martin, a shot further back after a 69.
Bidding for back-to-back major victories, Ricoh Women’s British Open champion Georgia Hall is still in the mix, sitting joint-fourth on nine-under following a 68.