"Skywalker leapt for him again, and this time Dooku met the boy's charge easily. They
stood nearly toe-to-toe, blades flashing faster than the eye could see, but Skywalker had lost his
edge: a simple taunt was all that had been required to shift the focus of his attention from
winning the fight to controlling his own emotions. The angrier he got, the more afraid he
became, and the fear fed his anger in turn; like the proverbial Corellian multipede, now that
he had started thinking about what he was doing, he could no longer walk.
Dooku allowed himself to relax; he felt that spirit of playfulness coming over him again
as he and Skywalker spun 'round
each other in their lethal dance. Whatever fun was to be had, he should enjoy while he could.
Then Sidious, for some reason, decided to intervene.
"Don't fear what you're feeling, Anakin, use it!" he barked in Palpatine's voice. "Call upon
your fury. Focus it, and he cannot stand against you. Rage is your weapon. Strike now! Strike!
Kill
him!"
Dooku thought blankly, Kill me?
He and Skywalker paused for one single, final instant, blades locked together, staring at
each other past a sizzling cross of scarlet against blue, and in that instant Dooku found
himself wondering in bewildered astonishment if Sidious had suddenly lost his mind.
Didn't he understand the advice he'd just given? Whose side was he on, anyway?
And through the cross of their blades he saw in Skywalker's eyes the promise of hell, and he
felt a sickening presentiment that he already knew the answer to that question. Treachery is
the way of the Sith"
- From RotS Novel
Here it is clear that Dooku was taken by surprise when Sidious encouraged Anakin to draw from the dark side in order to kill Dooku. Up until then, Dooku was stalemated with Anakin until the taunt when Dooku began to gain the upper ground. Sidious's advise caught Dooku off-guard and cost him the win once Anakin transformed into Zonakin.
And I'll throw this in too
"Skywalker's Shien ready-stance had been a ruse, as had his Ataro gymnastics; the boy was
a Djem So stylist, and as fine a one as Dooku had ever seen. His own elegant Makashi
simply did not generate the kinetic power to meet Djem So head-to-head. Especially not
while also defending against a second attacker.
It was time to alter his own tactics."
Falsely opening up with Shien also surprised Dooku