City thumped Feyenoord 4-0 at the De Kuip.
But De Bruyne insisted: "It was a tough ninety minutes. Feyenoord did not give up.
"A training game it was not. We started very well and if you can score those two, three goals, it will always be very difficult.
"We had to stay very focused and we did very well. We left little for Feyenoord to do, they could have scored two goals but we defended very well."