Olympic champion Justin Rose, who has twice been a runner-up at East Lake, was a shot off the lead alongside Rahm, Patrick Reed and Gary Woodland.
And look out for Jordan Spieth and Dustin Johnson - they're four shots back and teeing off together at 1:25.
Simpson was three shots ahead after completing his first 12 holes, but then a double bogey on the 13th left him with a 67 - the same score as Casey, who registered back-to-back bogeys on 12 and 13.
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Spain reported some 5,000 national police are already based there, and 3,000 to 4,000 more have been reported to have arrived . Catalonia represents a fifth of Spain's 1.1-trillion-euro ($1.32 trillion) economy and enjoys wide self-government.
Hitting back with two birdies in his last three holes again showed Day's tenacity and got him to T11 with two rounds to play but he will rightly feel it was a round that got away.
The American, who had shared the lead overnight at East Lake, was one under for his round after five holes and playing steadily as he approached the easiest hole on the course - the par-five sixth.
Spieth was among five players at 67 that included two other top seeds, Justin Thomas (No 2) and Jon Rahm (No 5). It was about as much as I can get out of a five iron. "Both missing our families, but all around, life's great", Casey said.
"I'm just trying to play the best I can and win a tournament". Simpson made par on the final five holes.
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The company delivers cold cans of caffeinated sodas to students and staff on campus within minutes of receiving an order via text. In a Facebook post, Caffeine Corner said, "Sounds like we're still in business".
The big worry is that despite being based in the United States for the last ten years, Casey has found it hard to close the deal when he has a shot at winning.
Casey seems to have done everything but win the last three years, with six top-5 finishes in the last eight FedEx Cup playoff events. Hopefully, ask me again on Sunday and I can give you the full explanation.
"The World Tour - it's going to happen one day and I think it has to", said the Northern Irishman. It's unbelievable how this golf course just bunches and you can't seem to get away.
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The charismatic Jacinda Ardern, who revitalised the main opposition Labour Party, also failed to muster sufficient numbers. Ardern and English were expected to maintain fiscal prudence, but to differ on monetary policy, trade and immigration.