Rory McIlroy had to watch from the sidelines for the first time in his Ryder Cup career as Europe attempted to overturn a daunting four-point deficit at Whistling Straits.
The Spanish pair were drawn to face Daniel Berger and Brooks Koepka, with Paul Casey and Tyrrell Hatton up against Dustin Johnson and Collin Morikawa and rookies Viktor Hovland and Bernd Wiesberger taking on Justin Thomas and Jordan Spieth in match three.
Speaking about McIlroy after submitting his pairings but before announcing them publicly, Harrington said: "You saw him out there after a tough day, he was out following those matches and supporting his team.
"He is very much a leader amongst his peers and I couldn't have asked more from him during the year, I couldn't have asked more from him on Friday.
"Yeah, the golf didn't go as well as he would have liked, but I'm not second-guessing him for a second in terms of his leadership and what he does for my team."
Harrington admitted his side were set for a tough weekend and would likely need to slowly chip away at the USA's four-point advantage.
"You can't just turn around and try to eat into a lead straight away in one session," Harrington said. "It would be lovely if it happened, but you can't think like that. You've got to do it slowly, one step at a time.
"We've got 20 points to play for and we've got to prepare ourselves for hopefully a long battle all the way through. If we are going to get this done, it's going to be a very tight one."
US captain Steve Stricker insisted he was taking nothing for granted, saying: "It's a great start. We are very happy with the start. But my message to the guys is Saturday is a new day.
"Let's just go out and try to win that first session again in the morning and pretend Friday never happened. Let's keep our foot down and continue to play the golf that we know we can play.
"We've had some things that came up and bit us in the rear in other Ryder Cups, so these guys know that and they are focused on coming out strong again."