DeChambeau was leading with five holes to play at the event in Germany, three of which were par 5s.
The wheels fell off for the 24-year-old on No. 15 when his second shot found the water leading to the first of two consecutive bogeys.
DeChambeau wasn't done there, dunking two more balls into the water on No. 18, carding a triple-bogey to finish the last four holes at 5-over par.
He then stormed off the green on No. 18, seemingly blowing off the champion McEvoy with a feeble handshake.
This is McEvoy's second win on the European Tour and first since 2004.
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