Lucas Moura hit a hat-trick for Spurs' comfortable win.
Siewert said, "I'm totally angry. The result doesn't reflect on the way we tried to play today. We had our opportunities to score earlier, and that's what makes me very sad today.
"That's why they're near the top of the table. With their quality, if you open the space too early for them then you're going to have problems.
“We observed all their matches and had an idea, and the players did quite well to be fair at the beginning of this game. This is hard to take."
He added: "We had great possibilities when Steve (Mounié) and Aaron (Mooy) came on, they had a direct impact. It was good for us to have possibilities like that, but in the end, the result is too harsh for me because the final score doesn't reflect the game at all.
"This is what I'm still learning about them. They are growing more and more into our ideas, but it's hard when you're playing against sides who are near the top of the table.
“If you make one mistake, they directly punish you. That's the quality of this league. It makes me proud to have a good view for next year."