Milan face Inter in one of the biggest derbies in recent years.
"The game is leaning towards Milan because they come into this appointment with evident physical sharpness and very high self-esteem," said Ancelotti in Il Giornale.
"But watch out for Inter, who never give in. The comebacks they had against Siena and Bari were incredible and a strong signal."
Ancelotti, whose Chelsea take on Inter in the Champions League, warned that he could change his mind if Nesta does not recover from his thigh injury. Following today's tests Milan confirmed the defender remains in doubt.
"A lot will depend on Nesta's presence. If he is not there, then Milan are about 30 per cent less effective. That's not just because of Nesta's strength, but the unity of his pairing with Thiago Silva."