Seven birdies negated a solitary bogey and helped Casey to an opening 66 in Hamburg and into a position he had not enjoyed on his home circuit since the 2006 Dunhill Links.
On six-under, the 42-year-old has a one-shot advantage over the Austrian Matthias Schwab and having finished third in his last start - the Tour Championship in Atlanta two weeks ago - is already an overwhelming favourite to win his second title of the year.
"It was a really good round of golf. Sure there were a couple of putts that slipped by but that is such a difficult course,” Casey said. “And here I stand even happier because the score doesn't really do it justice. That's one of the finest rounds of golf I've played this year. I couldn't have hoped for better; happy to be under par, let alone six-under.”
It was made all the more commendable because Casey has been suffering from a sore throat. "I'm not great, but it is what it is,” the world 17 said. “Maybe it’s a case of ‘beware of the sick golfer’.”
Casey’s countryman Ben Stow, Scotland's Robert MacIntyre and Germany's Max Rottluff are all on four-under, while the 2018 Masters champion, Patrick Reed, had to birdie the last three holes to shoot a two-over-par 74. That was one more than his fellow American, the world No 9 Xander Schauffele.
Stow, the world No 347 who normally plays on the Challenge Tour, was thrilled with his display. "There's a level of expectation you have to manage but this is a great opportunity,” Stow, 28, said. “Weeks like this can be a life-changer.”