"The score doesn't really do it justice, that's one of the finest rounds of golf I've played this year," Casey said. The English player finished a shot clear of Austria's Matthias Schwab, who had an eagle on the par-5 15th hole, then a bogey on the 17th before finishing with his fourth birdie.
"I had a four-week break and that came at the right time. I took some time off and worked on my game a bit, that seems to have paid off," Schwab said. Scottish rookie Robert MacIntyre, home favorite Max Rottluff and England's Ben Stow are two shots off the pace.
Sweden's Alexander Björk, Australia's Lucas Herbert and English player Ben Evans were at 3 under, just ahead of a group of four including 2016 Ryder Cup star Thomas Pieters. Xander Schauffele finished at 1 over after three bogeys in his opening nine, then a double-bogey on the par-4 10th, before the American picked up five birdies and a bogey in his last seven holes.
Defending champion Richard McEvoy was at 5 over.
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