Compared with the visual spectacle, bizarre techniques, and exaggerated clashes of the other battles, Kakashi and Kakuzu's fight was much more normal. Since they had already been fighting for a while, Kakashi had burned through most of his Chakra by now. But likewise, Kakuzu was far from being in peak condition. In terms of age, weakness, illness, and injury, the two were more or less evenly matched.
Kakuzu knew he had to do something. If he couldn't complete the mission assigned to him by the organization, then at the very least, he had to eliminate the enemy obstructing it. That wouldn't change the outcome of the failure, but at least it would give him something to answer to the organization with.
Based on Kakashi's performance just now and Orochimaru's evaluation of it, Kakuzu understood Kakashi's identity... Descriptions like "Copy Ninja" or "Kakashi of the Sharingan" weren't important. What mattered was that Kakashi was the disciple of the Fourth Hokage.
That meant Kakashi might have learned the jutsu that had once allowed the Fourth Hokage to dominate the shinobi world. It also explained how he had escaped right under Kakuzu's nose. In that case, Kakuzu naturally became extremely cautious about fighting Kakashi again... Everyone knew the Yellow Flash's ability to kill people in an instant. If Kakashi had learned even forty or fifty percent of his teacher's skill, then Kakuzu had to be sufficiently careful when facing him.
All right, Kakuzu was actually overthinking things a little. Kakashi's "Flying Thunder God" wasn't suitable for actual combat—not at least with his current abilities.
If he were facing the Yellow Flash, area-of-effect ninjutsu wouldn't work either. Against an enemy who appeared and disappeared unpredictably, close-quarters combat was actually the most effective. So, based on their own considerations, Kakashi and Kakuzu simultaneously chose to