There is so much on the line for both players as they can each qualify for the semi-finals.
Zverev knows a victory will see him through to the last four after he saw off Marin Cilic in the opening rubber.
However, Isner must win in straight sets and hope Novak Djokovic beats Cilic in the night session.
The American, who qualified for the tournament because of Rafael Nadal and Juan Martin del Potro’s withdrawal, has not won a set this week in London.
But he is determined to try and get out of the group stages with one big performance against world No 5 Zverev.
“Yeah, everyone knows how good he is,” Isner said.
“We’ve played quite a few times, as well. It will be a very tough match. There are definitely some things I need to improve.
“I’ll try to do that and implement them on Friday.
“But he does so much well. He serves very well, his groundstrokes are incredible. He’s a very tough out.
“There’s no slouches in this tournament, as I’m seeing out there (smiling).
“This isn’t your everyday tournament. It will be very tough. But I’m going to come out fired up.”
Zverev lost in straight sets to world No 1 Djokovic on Wednesday but he is desperate to bounce back.
“At the end of the day I don’t want to think about it too much because I will give myself the best chance I can to be in the semi-finals,” Zverev said.
“That’s where I think I’ll take it as a normal quarter-final match at a normal tournament.
“To be in the semi-finals here would mean a lot to me. For that, I have to play good tennis.”
Zverev will also qualify if he loses in three sets and Djokovic beats Cilic.
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