Lemke looked to have been falling behind, after entering day two as the joint leader, as he negotiated the front nine in level par, but five birdies in the final seven holes along with a bogey at the 15th moved him into the outright lead.
The 29 year old said he had patience to thank for such a strong finish and is looking forward to entering the weekend at Kytäjä Golf, in Hyvinkää near Helsinki, chasing the title for the first time in his Challenge Tour career.
“It was really good today,” he said. “I was patient all day and it paid off in the end which is a nice feeling.
“It was a little different from yesterday, definitely tougher, but I was pretty satisfied with how I played. It was pretty solid but I didn’t make the putts I made yesterday. Overall though, my game felt better so I'm happy with that progress.
“My putts didn’t go in in the beginning but it felt pretty good so they finally dropped towards the end. I definitely have momentum now and it’s good to follow up a low round with another good one so I feel I'm going in the right direction.”
Stewart, meanwhile, also carded a four under 68 to add to his opening 65 and, having made just one bogey in 36 holes, is feeling confident about his game.
Teeing off on the tenth, the 29 year old had a scorching back nine to thank for his good score on day two as he carded four birdies on the way back to add to a single bogey at the sixth, the only green he missed in regulation, and a single birdie on the front nine.
“Obviously hitting 17 greens in regulation was nice,” he said. “The front nine was pretty tricky actually when it rained quite a bit, it was playing really long and the wind was up so it was definitely playing more difficult.
“The putter was pretty solid again although I didn’t hole much but every second putt was a tap-in. I holed a really nice one though at the fifth. The only green I missed was in a tough lie right on top of a knoll so it was a tricky up and down but that was the only bogey I've made in 36 holes so I'm happy with that.
“Overall I was playing pretty solid, which is nice because it’s not a course that particularly suits me. I'm not the longest off the tee and that brings a lot of bunkers into play whereas if I had an extra 15 or 20 yards I’d be flying them.
“But I'm keeping it in play and I've only missed four fairways in 36 holes so it’s nothing spectacular but nothing bad, really solid.”
Roope Kakko was leading the home charge as he lay a shot further back on ten under par, alongside Jens Farhbring of Sweden and Welshman Stuart Manley.