Speaking as one who has limited experience with both plate armor and velociraptors, I think my odds are pretty damn good.
From what I can tell, your standard velociraptor weighs 45 pounds, and stands just 2 feet high at the hip. That's not very big...and he's got no armor plating to speak of. One good hit with the sword and he's out...even a glancing blow is likely to do some serious damage. Now I'm sure he's a quick bastard, but even so, he's going to have a hard time killing me quickly...my armor may not completely stop him, but it will slow him down, and biting through it will hurt him as well as me. Even animals that are trained to kill men have a hard time of it when the men are armed and prepared, contrary to what Rome: Total War would have you believe.
In fact, in your standard arena, one-on-one, I don't think the raptor has any chance at all, unless the man is exceptionally unlucky/unskilled. He may do some damage against the weaker guys before he goes down, but he's still going down, and I suspect that someone who really knew what they were doing would have no trouble at all. Packs of veliciraptors may be considerably more effective, depending on how well their pack hunting instincts translate to arena fighting.
Now if we're talking Deinonychus, (the 'raptor' from Jurrasic park) then the situation is a little different. Those suckers weighed in at somewhere around 160 pounds and stood 4 ft. at the shoulder. Still no armor plating that I know of, but sheer mass means it's going to be a lot harder to get in a killing blow, and they're going to able to do a
lot more damage to you in a short amount of time. I don't think I'd want to go at one of those with a sword...a spear would be a much better choice.
And it depends how deep the tank is.