The Soweto giants clinched the 2019 Shell Helix Cup after defeating the Brazilians 4-2 in an enthralling match at FNB Stadium in Johannesburg.
Middendorp said he had to substitute Njabulo Blom and Dumisani Zuma at half-time with Chiefs leading 4-0 as he had one eye on their Macufe Cup clash with Bloemfontein Celtic on Sunday.
"I said it before and I was not quoting somebody, I said it in a responsible way that, it is always very tough," Middendorp told SuperSport TV.
"It is not a friendly the game between Sundowns and Chiefs and in particular, Shell Helix put a lot of effort, congratulations to them for organising this game.
"We have seen it in the first half and over 90 minutes, a lot of aggressive moments, what do you expect? I said it, it's really competitive, I think it's fun," he continued.
"I never would have taken these two changes at half-time, Njabulo Blom played a good game, but he is involved tomorrow morning in the starting line-up too.
The German tactician was impressed by the two players' performances as Amakhosi lifted the unofficial trophy for the first time.
"Therefore I have to manage a little bit, 45 minutes, 30 minutes, taking out Dumisani Zuma at half-time was not necessary, he had a good game," he said.
"Both were really in control, supported the team in a proper way but if you have six possibilities to change it gives you the freedom to change and we took it and I think everybody accepted so far.
"It's a good step in terms of building your confidence, you win, if you're successful in the end, you get the three points, you win the Shell Helix Cup," he added.
"It's producing confidence and I think that's what it is."
Chiefs and their PSL rivals Celtic will clash at Free State Stadium in the City of Roses, Bloemfontein.