Lee bogeyed the 17th to drop alongside Clarke and Casey on one under, but Swindon’s David Howell, beaten in a play-off for the British Masters four weeks ago, and Australian left-hander Richard Green improved to three under.
Rain had been falling on and off all morning, but it did not bother the two leaders.
Both birdied the 409-yard first and long fifth, while Green also birdied the 548-yard fourth and Howell the 376-yard sixth.
They were on in front of Stenson, Norwegian Henrik Bjornstad, Frenchman Jean-Francois Remesy, Irish pair Eamonn Darcy and Andrew McCormick - one of three amateurs invited to play - and Sandy Lyle, who birdied the 10th and 14th.
Leaderboard:
4 under - Richard Green (Aus) (after 10 holes)
David Howell (10)
3 under - Fredrik Henge (Swe) (4)
Darren Clarke (9)
2 under - Henrik Bjornstad (Nor) (12)
Alex Cejka (Ger) (11)
Soren Kjeldsen (Den) (9)
Massimo Scarpa (Ita) (9)
Sven Struver (Ger) (6)
Yeh Wei-tze (Tai) (6)
Michele Reale (Ita) (4)
Jean-Francois Remesy (Fra) (12)
Des Smyth (9)
Adam Scott (Aus) (9)
Sandy Lyle (7)
Eamonn Darcy (7)