There's two or three possibilities if Thanos tries that. And someone better informed than me could answer which one it is.
1. Thanos burns Mantis's energy absorbtion out and overloads him. Might not happen, I think Mantis can control what he absorbs and he could just redirect it, I think.
2. Mantis absorbs all of Thanos's power. In which case, Thanos is ****ed.
3. If Mantis does have a limit, he could just start blasting the power back at Thanos, and expend as much energy as he absorbs, thereby keeping him under his overload limit.
But this third option would require Thanos to be an idiot. 😛