Grievous has never fought Ventress before. He may have sparred with her in a yet-to-be-revealed point during the war, but if you're referring to the Star Wars Tales version, you've got nothing. It has no value.
1) But does the starwars.com databank have no value?
2) Grievous is stronger than Asajj. In Dark Rendezvous it says Grievous is even stronger than Asajj.
3) Grievous is stronger than Asajj. In Labrinth of Evil it says Grievous was stronger than any of Dooku's other Dark Side apprentices.
4) Grievous is stronger than Asajj. That's why he's the General and she's but a Commander.
So fought or not he's still stronger.