Flekken kept a clean sheet as Bayer won 2-0 thanks to goals from Alejandro Grimaldo and Patrik Schick.
Hjulmand said afterwards: "I'm very, very happy and proud of the boys. We played exactly how we wanted to. We played with a lot of courage and calm on the ball when we had to.
"Of course, we also need a little bit of luck and a good goalkeeper. But at the end of the day, I think we played very well."
Indeed, on Flekken's performance, Hjulmand also said: "Mark has been improving ever since I came here. I've seen him improve from game to game, from training session to training session.
"He's trying to learn. You don't really recognise his ability to play with his feet. He's very, very comfortable on the ball. But, of course, today he also had saves that made a big difference.
"So Mark played fantastically, and I'm very happy for him because he's working very, very hard every day.