MIAMI—Reading John Isner’s serve can be as challenging as deciphering the registration tag on a car zooming by in excess of 100 mph.

So when Roger Federer faces reigning Miami Open champion Isner in Sunday’s final, the 20-time Grand Slam champion will use a multi-faceted approach to returning.

Isner: This Shot Separates Federer From Rest

“It's a tricky one,” said after schooling Denis Shapovalov in Friday night’s semifinal. “Sometimes you go with momentum. Sometimes you go with feel. Sometimes you guess maybe a little bit and sometimes you see it. It's a combination of all sort of things.

“And then some days you feel it better than others. Let's just say I know he's going to be T, let's say he told me that before the serve, that still doesn't mean I'm going to hit a return winner, because the margins are so slim when it comes in so fast and so high.”

Federer, who has beaten Isner in five of their seven career meetings, says putting a racquet on the American’s wrecking ball serve is the first challenge and getting to neutral position is the second priority.

“No. 1, you want to connect. Secondly you want to get in a neutral position, which is very difficult, because you know he's looking for his forehand or he looks to come in or try to take charge of the point,” Federer said. “Yeah, it's tough. Then you just hope that sort of the stars align, that you pick the right side, that he picks the wrong side, that maybe he misses a serve, that you can put him in uncomfortable situations time and time again, and at the end somehow you find a way.

"Yeah. He's definitely got one of the serves you can basically not read. It's that simple.”

Photo credit: Lindsey Godwin/Miami Open