One ahead overnight on 13 under par, the 36 year old Welshman went to the turn in 33 to maintain his bid for a first European Tour victory for nearly four years.
But Fisher, seeking a win which could take him seventh in The Ryder Cup race, birdied the long ninth and then started for home birdie-eagle to reach 15 under.
First the Volvo World Match Play champion, unexpectedly quiet since grabbing that title in Spain last November, hit his approach to three feet on the difficult 472 yard tenth.
Then he rolled in a 30 foot birdie on the 555 yard next - a hole which had already seen Dutchman Joost Luiten ran up a quadruple bogey nine, Argentina's Tano Goya a seven and Ryder Cup Captain Colin Montgomerie a six.
Dredge began his round with a 50 foot chip-in for birdie after driving into the trees and then going over the green and by picking up further shots on the third and fifth he opened up a clear gap over the field.
His putting then got him out of trouble at the seventh and eighth, but with Fisher mounting his challenge there was still a lot of work to be done.
Irishman Simon Thornton was in third place on 13 under, three behind, after a front nine 33 like the two players ahead of him.
One further back came former Open Championship winner Paul Lawrie, former European Open champion Kenneth Ferrie and former Open de France winner Pablo Larrazabal.
Fisher dropped his first shot of the day after driving into rough on the difficult 14th and Dredge took the opportunity to reopen a four shot gap, two-putting the long 11th and making his fifth birdie of the day at the 13th.
He was 18 under and Fisher 14 under with Lawrie and Thornton.