In addition to what deegen said, I have some reasons for being wary of Kyrgios in this one.
Here's a key quote from <a href="https://www.couriermail.com.au/sport/tennis/davis-cup-lleyton-hewitt-to-manage-nick-kyrgios-quick-turnaround-to-face-force-of-usa/news-story/79f68baae79328462c39d0b52d4012a2">this</a> article about Kyrgios's travel woes ahead of the Davis Cup:
"Kyrgios said he had 'jarred' his knee [against Zverev] when attempting an overhead and while he said after the match he felt no pain he would no doubt be relieved to wake up with no soreness."
Here's some context about their first meeting in 2015 from <a href="https://www.desertsun.com/story/sports/tennis/bnp/2017/03/16/nick-kyrgios-roger-federer-match-comes-baggage/99284262/">this</a> article previewing what would've been their matchup at Indian Wells:
"The first time they played was on the clay courts in Madrid. It was the birthday of Federer’s twin boys, he said Wednesday, and he blew three match points to allow Kyrgios to rally for the win."
More from that article:
"Federer, for his part, still remembers that loss to Kyrgios on his boys’ birthday.
'I'm happy my family is here,' he said. 'It's not anybody's birthday, and I could just focus on playing tennis. Of course I'd like to get him back.'"
Despite Federer's shaky play against Berdych, he gutted out a win. I imagine that winning when you don't play your best is a confidence booster. Now he's up against a sometimes self-destructive, possibly injured opponent who's entering the match on less rest and with questionable motivation. Regarding that last point, here's Kyrgios talking about his match today in the first article I linked to:
"'I’m just excited to give it my best shot,' Kyrgios said.
'If it’s good enough, it’s good enough. If it’s not, it’s not. Then we focus on Davis Cup next.
'I don’t know if my chances are great to play Federer and possibly Nadal, two of the greatest of all-time.'"
Maybe Kyrgios has finally learned how to keep things on even keel and the above quote is just indicative of that, but I'm inclined to see those words as a guy in his mental armor with respect to this match. I'm taking the GOAT -2 here.
Best of luck.