The league-leading Blues have won six of their seven outings so far this term and Ivanovic feels it is important for the group to keep up that winning mentality against former Stamford Bridge boss Andre Villas-Boas' Spurs after the international break.
"The international break is a good time to enjoy our position but there is a very important period coming up for us after the break," he said in the Fulham & Hammersmith Chronicle.
"We have some very tough games and they will be a great challenge for us. At the moment it's OK but the next two games will be very, very tough.
"It's always very difficult to play the game after the international break, but especially when it's big opponents like Tottenham away.
"Our games against Tottenham were very interesting and tough for both sides even before Andre [Villas-Boas] went there. Now it is especially so for both sides with our ex-manager.
"It will be difficult in one way and in the other way we know what is the idea and what we can expect of them.
"This game will be decided by things like when you are back from your international duty and how tired you are - the small details. So it's going to be tough for us."