Amakhosi succumbed to a brace by Judas Moseamedi as they suffered a 2-1 loss to Eric Tinkler's side at the Mbombela Stadium.
"Congratulations to Maritzburg United, I think a deserved win, there's no doubt about it. After we conceded 1-0 in the first-half, absolutely challenges with our defence despite we were sometimes two against one, three against one," Middendorp told SuperSport.
"A bit surprisingly, on the other side in terms of our combination game through the midfield it was not really a factor as it could be, should it be, how we can do it, good," he went on.
"Then of course you equalise, then you give it away again, a little bit surprising with the quality we should have. It was not definitely the performance what we have seen today, what we are capable of."
The former Maritzburg United coach felt his players underestimated the Team of Choice and were already thinking about the final.
"Always world wide something when you're going to the game, you're just not really... bring your concentration to the point where it should be. You're already in this game playing [Mamelodi] Sundowns on the 14th [December].
"We spoke about it several times, I said, "Please don't underestimate [Maritzburg]. Please make sure that we're first taking this game before we think about the next one". That's a normal way, it's nothing unusual, it happens world wide. I was thinking we would be able to handle it but opposite we failed here," he added.