ST OMER, June 15, 2013 (AFP) - Thornton's third round six under par 65 featuring two eagles including his ace at the par three 11th left him with a share of the lead with South African Tjaart van der Walt and France's Baptiste Chapellan.
This trio, on a combined total of four under par 209, head into the final round one shot clear of Briton Steve Benson, with German Max Glauert, England's Chris Lloyd and Frenchman Victor Riu were all a further shot back.
After his hole in one heroics in breezy conditions Thornton, who was born in the northern English city of Bradford, said: "That was obviously pretty good.
"I just played really well, hit some good shots and in weather like that you just have to keep straight and that's what I do well," said the 36-year-old.
"There were times when you could have hit any one of five clubs because the wind was unpredictable, but you just have to pick one and trust it.
"I holed about a 30-footer for eagle at the ninth and then the hole in one.
"I was in between a seven iron and an eight iron and I hit a nice smooth seven and it was all over the flag, one big bounce and it just rolled in, it was really good shot.
"There were a few high fives and shouts, it was my first one in a professional tournament so I was delighted."
England's Robert Dinwiddie, who had a share of the lead at the start of the day, dropped back to three shots off the pace after a round marred by five bogeys for a 74.
Leading third round scores at the St Omer Open on Saturday (GBR & IRL unless stated, par 71):
209 - Tjaart Van Der Walt (RSA) 67 71 71, Baptiste Chapellan (FRA) 72 68 69, Simon Thornton 74 70 65
210 - Steve Benson 75 65 70
211 - Victor Riu (FRA) 68 71 72, Max Glauert (GER) 77 65 69, Chris Lloyd 73 68 70
212 - Jamie Elson 75 69 68, Robert Dinwiddie 72 66 74, Pelle Edberg (SWE) 74 69 69
213 - Brooks Koepka (USA) 75 70 68, Jeppe Huldahl (DEN) 73 67 73, Agustin Domingo (ESP) 74 71 68
214 - Andrea Pavan (ITA) 74 70 70, Daniel Im (USA) 73 72 69, Michael Jonzon (SWE) 74 70 70
215 - Mikael Lundberg (SWE) 73 71 71, Gareth Shaw 76 69 70, Daniel Gaunt (AUS) 70 70 75, Scott Henry 72 69 74, Rhys Davies 77 67 71, Daniel Vancsik (ARG) 73 67 75, Justin Walters (RSA) 71 69 75, Alexandre Kaleka (FRA) 73 71 71, Francois Calmels (FRA) 73 71 71
216 - Tyrrell Hatton 75 67 74, Chris Hanson 71 68 77, Daniel Brooks 69 71 76, Richard McEvoy 71 71 74, Edouard Dubois (FRA) 71 74 71, Simon Wakefield 71 68 77