The Englishman won $2.02 million at the World Match Play Championship last year after beating former PGA champion Shaun Micheel by a record 10 and 8 in the final.
Top-seeded Casey starts his defense Thursday against American Jerry Kelly, who is the lowest-ranked player (No. 55) in the 16-man field.
Despite a summer slump, Casey's game is improving just in time.
'Winning this week would turn it around. I would be a very happy man because I've never defended a title successfully,' Casey said Wednesday.
The 30-year-old Casey, who played golf at Arizona State, plans to reapply for his Tour card next year and will play in at least the 15 events required for membership.
Ernie Els, the highest-ranked player at Wentworth this week at No. 5 and a six-time winner of the title, begins against Colin Montgomerie. Third-seeded Padraig Harrington, the British Open champion, plays Anders Hansen of Denmark, winner of the BMW PGA Championship in May.
The other matches are: Rory Sabbatini against Soren Hansen, Justin Rose against Hunter Mahan, Henrik Stenson against Woody Austin, Niclas Fasth against Andres Romero, and Retief Goosen against U.S. Open champion Angel Cabrera.
Played at the 7,308-yard, par-72 West Course, all matches are 36 holes.
Casey won his first event of the year in Abu Dhabi but has not won since, although he finished all four majors this year and was tied for 10th in the Masters and the U.S. Open.
Kelly has had equipment problems this year after a successful run of top 10 finishes -- he tied for fifth at the Masters and seventh at the U.S. Open. He has been struggling to find a driver he likes and was experimenting with three on the practice range Wednesday.