The 24-year-old Finn holed from long distance on the way out and after turning for home, with two runs of four straight birdies at Mont Choisy Le Golf.
He leads by two shots from playing partner Oliver Bekker with Frenchman Pierre Pineau third on seven under.
Pineau set the early pace when he birdied five of his last seven holes to sign for a seven-under 65 but Valimaki reeled him in after catching fire with his putter - firing four birdies in a row from the fifth and again from the 12th.
The pick of the bunch on his five-under front nine was the 55-footer he holed for birdie at the sixth, before another long putt dropped at the next.
He gained a further shot on the par-five 12th following a nice chip to tap-in range and joined Pineau at the summit after a smart up-and-down from wet sand at the 13th.
He was out on his own after taking advantage of the par-five 14th with a third birdie in a row and a long, uphill putt for a two at the 15th gave him a two-shot cushion.
There was a slight wobble when he found rough off the tee and sand with his second at the 16th but he got up and down to keep his card clean before a closing birdie took him to double digits in the red.
Valimaki said: "It was a really nice round. I putted really good, made some longer ones - I haven't made them the last couple of weeks, so it's nice to get it rolling.
"Oli (Bekker) played a really good round as well, so we enjoyed the day. I feel like I found the rhythm and the speed for the greens and that was the key."
Like Valimaki, Bekker was bogey-free, posting eight birdies in his 64.
Englishman Bradley Bawden was the leading British player after starting out with a four-under-par 68, with his compatriot Matthew Southgate a shot further back.