Saturday's round was played in damp, chilly conditions with a very wet golf course. Overnight rain drenched The Belfry and made it play much longer in round three.
Reigning U.S. Open champion Michael Campbell posted a 4-under 68 to move into second place at 11-under-par 205. Darren Clarke is one stroke further back at minus-10 after a 70 in round three.
Stephen Gallacher and Johan Edfors share fourth place at 9-under-par 207 after they both shot 70 in round three.
Casey stumbled to a bogey on the par-5 third to drop to 10 under and a share of the lead with Campbell, who birdied each of the first three holes.
The 28-year-old Casey managed to recover that stroke with a birdie on the par- 3 seventh. He made it two straight with a birdie at the eighth.
Casey faltered to another bogey at 10 to slide back to minus-11 after he found water next to the green with his approach shot. He poured in a long birdie putt on the 13th for the second straight day to atone for that error.
The Englishman birdied No. 15 for third consecutive round to move to 13 under, three clear of Campbell and Jarmo Sandelin. Casey came right back with a birdie on 16. However, he tripped to a bogey at the last to slip back to 13 under.
Casey owns the 54-hole lead for the seventh time in his European Tour career. He has gone on to win two of the previous six events, including the 2005 TCL Classic.
'I am very happy to be playing with Michael (Campbell) tomorrow,' Casey admitted. 'He's a major champion and I've learned a lot from him and it will be tough again. Darren is a good player too and if he is hot tomorrow, then he will be a very big danger.'
Campbell parred four straight after birdies at the first three holes. The New Zealander got within one of Casey's lead thanks to a birdie on the eighth.
He stumbled to a double bogey at the next though to fall to minus-9. Campbell got one of those strokes back with a birdie on No. 14. He moved back to 11 under as he birdied the 17th.
Campbell has a side bet going with Casey this week. That extra incentive has helped him stay in contention this week.
'Paul went through a bad patch there last year, but he's got so much talent. I was just telling him, be patient with yourself. We started a betting system, he and I,' Campbell stated. 'When I'm out there playing and I see certain guys up there ahead of me, it makes me try a little bit harder.
'This week is a prime example between myself and Paul. We've had a bit of a thing going on, and it's really helped me a lot these last three days, let me tell you.'
Clarke, who entered the round in second place, was 2 over par for his round through 11 holes after two bogeys, two birdies and a double bogey.
The Ulsterman birdied three of four holes from the 12th to get back to minus-9. Clarke posted his final birdie at the 18th to end three back.
Sandelin is alone in sixth place at 8-under-par 208 after shooting 70 on Saturday. Peter Hanson (72), Tom Whitehouse (69) and Oliver Wilson (68) share seventh at minus-6.
Anders Hansen, Jonathan Lomas, Graeme McDowell, Peter O'Malley, Ian Poulter and Gary Emerson are one stroke further back at 5-under-par 211.