Goals from Kyle Benedictus, Steven Milne and Kevin McBride gave the Dark Blues a commanding lead and despite Leigh Griffiths pulling a goal back in injury-time, it was no more than a consolation effort.
"We got what we deserved. We didn't perform to the levels we have been doing of late," said Fenlon.
"We haven't had a performance like that all season so it's difficult to explain.
"Dundee worked harder than us but we have to take it on the chin and it's hard to be overly critical of the players.
"We spoke about it at half-time because we were lucky to go in just 1-0 down as it could have been more. But we didn't start as well as we wanted to and all of a sudden we were 2-0 down and it was all uphill from there on.
"We kept at it but didn't do enough to take anything from the game. We've got to make sure that we concentrate on our own form. We didn't do that today, which was disappointing.
"You don't want to take too much away from Dundee because they've been battling hard all season as a team and deserved this win. They passed the ball well at times and scored some good goals."