Spurs blew a 2-0 lead at Falmer stadium to lose 3-2 on Sunday.
Postecoglou was asked about Spurs being too cocky, but insisted: "Look, it's not a couple of positive results. It was five wins, let's just keep things in context here. I understand the extremes of what we do. What I'm saying is that we didn't do the basics of everything we've done to get to this point, good, bad or otherwise.
"Even when we didn't win at Newcastle, we played well and fought hard to the last second to win that game of football. We didn't do that today and every time we've played that's been an element of our game but I didn't see that today. Why has that happened?
"The reasons, we can go into a million different aspects and in the cold light of day there may be some logic to it and it may be what you're saying. Again for me that's irrelevant. It's for me to address and fix and the responsibility falls at my feet and that's what I need to do."
On the expected returns of injured pair Richarlison and Heung-min Son, he added: "No, it's got nothing to do with it, absolutely zero to do with it. If we were missing them then we wouldn't have played like we did in the first half, or like we have in the last few games. So it's got nothing to do with it.
"Like I said, there's a certain level of competitiveness you need to bring in every contest, irrespective of the level, and we didn't do that in the second 45. And rightly so, we got punished for it."