Well, if there is one thing Sans is good at it's avoiding things. He reflexively teleports out of the way of Greg admittedly shocking strike - straight up attacking him is not something Sans was expecting, even with Greg acting so unreal.

The betrayal is there on his face when he reappears, for a solid moment before it shifts into his own biting sneer.

"Your a lover, not a fighter, Greg. Still can't aim for shit."