Even though the duo advanced and retreated during the time they fought against each other, they also clearly understood that neither side had used their full strength!
There was a big reason why Happy did not use his full strength. After all, not only could he not use the Murong Clan's martial arts, he could also not use Eighteen Dragon Subduing Palms or Ten True Dragon Styles. Besides, he also could not use a lot of his equipment. He could only activate his qi realm up to Life and Death Realm, so his strength could be said to be put under a great restriction.
Even so, with Nine Yang Divine Skill's help, he did not have any problems with handling the mysterious man in black.
Instead, he was a little curious as to why the man in black could retain his strength when he faced Bright Sword Technique and Flying Dragon Sword Technique. What he showed already made him stronger than anyone else aside from One Emperor.