Watson, 39, started the day six shots behind overnight leader Casey, but took the outright lead with a birdie on the 18th to finish on 17 under.

England's Casey, who shot a 62 in the third round, struggled to keep pace.

His round unravelled with back-to-back bogeys on the 16th and 17th, as he signed for a 72 to finish on 14 under.

American Stewart Cink had the round of the day with an eight-under 62 to tie for second with fellow Americans Beau Hossler and JB Holmes, and Casey.

Northern Irish four-time major winner Rory McIlroy finished on 11 under after a 67, while US Open champion Brooks Koepka was tied for 19th on nine under.

"It was a fun round of golf and I got lucky that he [Casey] didn't have his best stuff," two-time Masters winner Watson told Sky Sports after his third win of the year.

"It's one of those years where you want it to keep going, you just don't want it to end."

Notifications: Sign up to get golf news sent to your phone