Both start in base
Setting Los Noches's main throne room.
Base vs Base: Tousen blitzed Grimmjow at least once already (plus cutting through his Hierro casually) and portrayal places him at a pretty high standing. So Tousen wins handily.
If Grimmjow releases, Tousen could beat him with his Bankai (with high diff.) or simply put on his Vizard mask and thrash him.