He didn't, Dooku was told to not defeat him. Sidious said he would step in if something which he doubted would go wrong. Dooku changed his form when he fought Anakin and he was no longer fighting to win, just to keep Anakin busy. Of course when things did go wrong against expectations Sidious did not step in and let Dooku die.
If you read Dark Rendezvous and Labyrinth of Evil, they give insights into Dooku's growing hatred and contempt for Anakin. Also, he underestimates the young Jedi, thus causing his own fall. ANd he did change his fighting style mostly, adopting a Makashi/SHien hybrid which mad him spin like a top and made his defense terrible.