Lucius would have been surprised and worried that Buffy had gotten so sloppy so early on in the fight. On this occasion though he wasn't analysing everything around him, he was in a fight and so was wrapped up in it. He saw the opening and automatically took it, exactly how he had been trained to but as Daniel he had ignored it unless taking them out was the best option.
He caught her wrist, stepped forward and used her own momentum to land a punch directly to her sternum. Not only would it hurt like a bitch, it would also wind her. For Buffy's sake it would be a good idea to end this fight before she got herself majorly hurt and so he used her emotional disorientation to his advantage. He kicked her legs from under her at the same time as sweeping his arm in the opposite direction and hitting her shoulder. It was far cleaner and less obvious than the sweeping kicks that Buffy was so fond of.
"You should never fight out of anger, Buffy. It clouds your judgement" Lucius lectured her