Woods, seeking his 80th win on the PGA Tour and his first since 2013, left himself a long second shot into the 18th green, and could only cajole a tricky 11-metre putt within about a foot of the hole, and tapped in for the par.
Casey shot a closing six-under par 65 to finish at 10 under to win by one shot from Woods and fellow American Patrick Reed on the Copperhead course at Innisbrook Resort in Florida.
Thomas, who has two wins this season, has attracted the most money to win the Masters at the Westgate, including a five-figure wager at 15-1.
Woods said in January, when he returned to the PGA Tour at Torrey Pines, that the early stages of his latest comeback would all be aimed at Augusta National, where he has won four green jackets, but hasn't played since 2015.
The 40-year old Cheltenham-born Casey last won on the PGA Tour in capturing the 2009 Houston Open and in the intervening period has contested 155 Tour events before being handed the Valspar "Paint Brush" trophy.
"I've been watching him swing and he's really got it in good positions, " said Ernie Els, the Hall of Famer who has been friend and rival to Woods for 20-plus years.
Johnson and Thomas are each 6-1 and are the only golfers with better odds than Woods. Casey knows that the next generation of tour stars has already arrived, which makes his accomplishment all the more meaningful.
But victory was not to be for the 14 major victor and holder of 79 trophies. He then needed to convert from 39 feet on the 18th, but it didn't fall, and Casey escaped with a one-stroke victory. "At least I'm there with a chance going into the weekend".
He drained a putt from 22 feet for birdie at No. 10 underneath an overcast sky similar in color to his dark gray slacks and cap, contrasting Woods' bright pink Nike shirt that helped him stand out in a crowd even more than normal.
"I've been here before a few times, so I felt very comfortable".
Just imagine how excited people will be if Woods, looking fitter than he has in years following four back surgeries, gets into contention at next month's Masters, where he was taken the green jacket four times. My game is sharp and unfortunately I didn't hit the ball close enough to make shots. "I need to play a good, solid round, clean card again and we'll see what happens".
But over 54 remarkable holes at Valspar, with expectations starting low and swelling higher, Woods has proven he can still play.
"It was a very hard week - tough conditions on a tough golf course, very demanding and I thought I held up really well". "I was kind of getting a little frustrated, seeing the scores were decent". Still, as I said yesterday, win or 15th, this was a great week for him. I haven't done that in an bad long time.