The 39-year-old has been waiting since 2001 for his breakthrough win after 12 trips to the European Tour Qualifying School and years on the European Challenge Tour.
"It was good. I was a little disappointed with the back nine, missed a few chances coming in on the last four holes or so, but the change in conditions made it quite tough", McEvoy said on the European Tour website. I'm absolutely over the moon. What a massive step forwards for him, as he won €333,334, whilst the total prize fund for the Challenge Tour event was just '€210,000. "I'm a little bit lost for words".
"I've tried to enjoy my golf as much as possible".
"I prevailed last week so fingers crossed I can finish it off again tomorrow", he said.
Dechambeau and McEvoy started the final round as co-leaders, but Dechambeau fell away with a closing 6-over 78 in Hamburg, Germany.More news: NVIDIA announces GeForce Gaming Celebration ahead of Gamescom 2018 with spectacular surprises
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"I apologize to Richard McEvoy and the fans for my brevity on 18", he wrote.
It was a late finish at the Green Eagle Golf Courses because play was suspended as lightning flashed ahead for nearly three hours during mid-afternoon. He did the former, rolling in a final birdie on the par-five 18th - a relative surprise given his putter hadn't been the strongest part of his game during the final round.
Rather than congratulate McEvoy with any sort of enthusiasm, Dechambeau brushed his rival with a weak handshake and walked angrily past him off the course.
Masters Tournament champion Patrick Reed was then at seven under alongside fellow Augusta victor Charl Schwartzel, Scot David Drysdale and England's Matthew Nixon, one ahead of a group containing DeChambeau. But I fought hard, I believed and even at the last I overpowered my caddie to lay it up and give myself the best opportunity to make birdie and managed to do it.