Spurs, on the back of defeat to Bournemouth, need to overturn a 1-0 deficit to reach the final.
Sissoko said: “This week will be the biggest of the season for us, because if we beat Ajax we can be in the final of the Champions League and that would be something amazing.
“At the beginning of the Champions League, no one expected us to be in that position. So if we win and go the final it will be a dream for everyone.
“We have to believe in ourselves, because we can do it.
"If you are in the semi-finals of the Champions League and the next season you are not in the Champions League, it will be a bit painful, but I don't think about that."