Lets put it this way He standing there talking down to Drax and all of a sudden Drax belts him and Silver Surfer tries to stop Drax when Thanos come up behind Silver Surfer and uses his Power Cosmic on Silver Surfer and when Silver Surfer gets up he is pissed and he joins Drax in the battle against Thanos saying this might be the only chance I have against Thanos 😆 because it would.
Silver Surfer hasn't hit Thanos with all his might, he never does that in comics. But in these theoretical fights we assume that he fights at his very best ability, which includes helluva amount of superspeed, telepathy, matter manipulation, amping his abilities to god-knows-how-many-folds and energy bolts. If SS gives everything he's got he'll give Thanos a run for his money. SS might even just take Drax's power gem and use it.
But as told, it's no use fighting since Thanos cannot be slain.
I doubt Thanos still has the Death curse, I'm sure he can be killed, he and Death are on decent terms now. In fact I think in Thanos #1 he said he isn't "easily" killed, which sort of implies he can be. Although maybe its been stated somewhere he still has it, in which case someone feel free to correct me.
Here are some battles of Silver Surfer. http://www.geocities.com/bruno_nojunk/type-fights-16.html