Cooper, also fastest on Tuesday, was one second ahead of Frenchman Pierre Yves Bian (VAS Engine Paton) after setting a best lap of 110.608mph in dry conditions around the 8.9-mile course on Thursday morning.
Magherafelt man Paul Jordan was third quickest on the PreZ Racing Kawasaki, 0.7s further back, with Joe Loughlin (Paton) next ahead of Jeremy McWilliams on the IFS/Bayview Hotel Paton.
Yorkshireman Jamie Coward completed the top six on his KTS Kawasaki ahead of Michael Dunlop, who is riding a McAdoo Kawasaki.
Fermanagh’s Lee Johnston was the top Aprilia rider on the new RS660 machine in eight place, almost six seconds down on Cooper’s Kawasaki.
1 R Cooper (Kawasaki) 110.608mph
2 P Yves Bian (Paton) 110.213mph +1.044s
3 P Jordan (Kawasaki) 109.92mph +1.827s
4 J Loughlin (Paton) 109.484mph +2.997s
5 J McWilliams (Paton) 109.432mph +3.136s
6 J Coward (Kawasaki) 109.002mph +4.300
7 M Dunlop (Kawasaki) 108.665mph +4.452s
8 L Johnston (Aprilia) 108.417mph +5.899s