The 37-year-old carded nine birdies at the Waialae Country Club in Honolulu as he set the early pace alongside American Webb Simpson.
Casey is usually in the Middle East at this time of year but has opted to play at Waialae for the first time in a decade and appears to have made a smart decision.
There was hardly any wind on Thursday along the shores down from Waikiki Beach and scoring was low.
Casey began his round at the 10th and kicked off with a birdie, before going on to pick up a further shot at 14.
And the three-time member of the European Ryder Cup team then charged up the leaderboard with a run of five birdies in a row on 17, 18, 1, 2 and 3.
He also birdied five and, although his only blemish of the day came when he made a five at the par-four sixth, he bounced back with a birdie at nine, his 18th.
Simpson also played the back nine first but he was just one-under at the turn, before making seven birdies as he shot 28 for his front nine and joined Casey on eight-under.
Camilo Villegas had a 63, shooting 29 on the front nine before cooling down somewhat, while Jason Day and Matt Kuchar are among those at 65.