In The Time of Contempt, when Geralt fights Vilgefortz on the isle of Thanedd, it is implied that Vilgefortz is not human, or at least not merely human (emphasis mine):
A few weeks later, having been healed by the dryads and the waters of Brokilon, Geralt wondered what mistakes he had made during the fight. And came to the conclusion that he hadn't made any. His only mistake was made before the fight. He ought to have fled before it even began.
The sorcerer was fast, his staff flickering in his hands like lightning. Geralt's astonishment was even greater when, during a parry, the staff and sword clanged metallically. But there was no time for astonishment. Vilgefortz attacked, and the Witcher had to contort himself using body-swerves and pirouettes. He was afraid to parry. The bloody staff was made of iron; and magical to boot.
Four times, he found himself in a position from which he was able to counterattack and deliver a blow. Four times, he struck. To the temple, to the neck, under the arm, to the thigh. Each blow ought to have been fatal. But each one was parried.
No human could have parried blows like that. Geralt slowly began to understand. But it was already too late.
However, as far as I know this is never explained, neither in The Time of Contempt nor in any of the following books in the series. What is Vilgefortz, if not merely an extremely skilled human?