The suit then lasts about three or four exchanges of them hitting each other, while it is melting and then breaks. Earlier in the fight it's commented that the pain of being in the corona/atmosphere (not sure what the right word is) of the Sun is extremely painful to them both and is making Thragg sluggish, made all the worse when they both take a dip in the actual Sun and get their skin burnt off.
I think the intention of the scene, along with Mark monologuing about how he's fighting for something outside of his own strength (unlike Thragg) is intended to highlight that Robot's suit was a key factor. It may have been moments but it was several exchanges where he was protected and Thragg was not.
He also does get his forearm bone snapped pretty clean by a punch from Thragg once the suit is off.