Frank de Boer's men trail leaders PSV in the Dutch top flight ahead of their return to domestic action against Groningen at the Amsterdam Arena on Friday.
Ajax have spent the bulk of their winter break in Doha, with the trip including a 2-0 friendly win over Schalke 2-0 on Saturday.
While an injury to Joel Veltman cast a cloud over their preparations for a return, Klaassen feels Ajax are in a great position to reel in PSV during the second half of the campaign and win a fifth consecutive title.
"When you see this training camp and this match, I am very confident that we will be champions again," he told De Telegraaf.
"We also agreed this week that I and some of the other guys have to stand up as leaders. That gives us a good feeling.
"We need to be right and can not relax. Also, because we are four points behind, we have to create something good against Feyenoord and Vitesse [in the next few weeks].
"But first against Groningen on Friday."
