The consistent Englishman signed for a 5-under round of 65 Saturday at East Lake Country Club to establish a two shot advantage over Americans Kevin Kisner and Xander Schauffele.
Casey, a perennial contender at the FedEx Cup playoffs but without a win to his name, sits at 12-under with 18 holes to go and a legitimate chance to also win the season long race and its $10 million bonus.
The top five ranked players at the start of the week are all guaranteed to win the FedEx Cup if they win the tournament but Justin Thomas is the best of them at five shots adrift of Casey.
Jon Rahm is a further stroke back while Dustin Johnson and Jordan Spieth are eight shots off the pace and T13. Marc Leishman is out of the reckoning at 13 shots behind.
Jason Day is on the fringes of contention after another frustrating round which saw him drop shots late in the day to fall off the pace.
Five front nine birdies against a lone bogey had him within three shots of the lead but, as he has on each of the first two days, he battled on the run home.
Eight pars and a double bogey see him head to the final round 6-under and six off the pace though he has shown enough this week to suggest a super low round is not out of the question.
Day’s good golf has been outstanding but clumsy mistakes have cost him, a wild drive at the 16th hole on day three a prime example.
After hitting his ball against the out of bounds fence right he had almost no swing and was forced to chip out but still in rough and trees.
His third found a greenside bunker and he was unable to get up and down, the resulting double bogey a frustrating continuation of a theme for the week.
Each day the former World Number One has gone to the turn under par and each day has been unable to go on with the job on the back nine.
However, Day has built a reputation as one of the game’s most tenacious players in recent years and his position in the 30-man field at East Lake is testament to that.
He started the four play-off events in 49th position on the FedEx Cup and not guaranteed to even qualify for the season ending tournament.
But he produced some excellent golf under pressure in the first three events and is now projected to finish just outside the top 10 in the final standings.
At the top of the leaderboard Casey is in prime position to claim just his second PGA Tour title.
The Englishman has long been considered one of the game’s best players but despite 13 wins on the European Tour his only victory in the US came at the Shell Houston Open in 2009.
A series of injuries, including a dislocated shoulder after a snow-boarding accident in 2012, slowed his momentum for a number of years but he has been building back to his best since 2015.
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