Fellow Englishmen Luke Donald and Nick Dougherty are also at 5 under par after both birdied the 18th hole, while South African Andrew McLardy set the pace with a bogey-free 67 earlier in the day.
The leaders are one shot clear of a group of four players who stand at 4-under-par 68, including defending champion Angel Cabrera of Argentina.
Joining Cabrera in fifth place at the Wentworth Club's remodeled West Course were David Howell of England, France's Raphael Jacquelin and Irishman Peter Lawrie.
Lurking one shot further back is a pack of 11 players at minus-3, including world No. 6 Ernie Els of South Africa, one of the course's designers, and 1994 champion Jose Maria Olazabal of Spain.
A new and tougher course design produced the star-studded, first-round leaderboard, peppered with a lot of the field's high-profile players.
'It was a good test, but if you played properly today you'd shoot under-par,' said Els, who orchestrated the West Course re-design. 'The old course, [with] conditions like these, I think you would have seen a 64. Today was your scoring opportunity.'
McLardy collected three birdies on the front nine and two on the back to become the first player to finish at 5 under. His final birdie, at the par-4 16th, came on a 40-foot putt.
Dougherty was the second co-leader to close out his round. After playing the front nine at even-par, he earned his 67 with a bogey-free back nine that included five birdies, including two straight at the 17th and 18th holes.
Donald's bogey-free round began with three birdies on the front nine, including back-to-back birdies at six and seven. He added two birdies on the back nine -- including his birdie at the 18th -- and finished the round having hit all 18 greens in regulation.
Like many others, Donald enjoyed the lift, clean and place rule effected by recent heavy rain.
'It was target golf today,' said Donald. 'I think that is the first time this year I have hit all 18 greens in regulation.'
It looked like that trio would share the overnight lead without any more company until Casey, who has top-5 finishes in each of his last four European Tour events, caught fire on the final two holes.
Casey had been just minus-2 before his birdie on 17 and the eagle at 18.
'Typically this has not been a golf course I've played well,' Casey admitted. 'I think I'm excited about the changes...and maybe that's what I needed.'
Joining Els and Olazabal in ninth place were Francois Delamontagne, Jamie Donaldson, Kenneth Ferrie, Garry Houston, Robert Karlsson, Paul Lawrie, Ian Poulter, Phillip Price and John Wells.
World No. 3 Retief Goosen leads a group of 12 players knotted in 20th place at 2-under-par 70 after and up-and-down round that included five birdies and three bogeys.