Roger Federer admits he "wishes" he knew when he'd retire but will continue to play as long as his body and mentality allows him to. (2:08)

MIAMI GARDENS, Fla. -- John Isner doesn't like his chances of winning the Miami Open men's final on Sunday (1 p.m. ET, ESPN2). It's not that Isner lacks confidence, or ability. His serve is the most dreaded weapon in tennis. It's just that Isner can do calculations. It's not for nothing that he put in his four full years at the University of Georgia.

"I was just going on pure math," the 33-year-old veteran said on Friday after defeating Canadian teen Felix Auger-Aliassime 7-6 (3), 7-6 (4) in the semifinals. "I have only won one of these (Masters tournaments) in my whole career, and I have probably played a hundred of them (actually, it's 78). You just crunch those numbers."

Another number-crunching exercise puts Isner's prudence into perspective. Roger Federer, who will play Isner for the title, has played in 124 Masters tournaments, been in the finals of 50 of those and owns 27 titles. Admittedly, the chore facing Isner looks monumental. But he has a hole card of sorts: "I believe, personally, that any tournament I enter I can win because of how disruptive I can be, and because of how well I can serve at times."

Isner earned that lone Masters title he cited at this tournament last year, when the Miami Open was still held 20.7 miles away on Key Biscayne. The defending champion has weathered the change of scenery to Hard Rock Stadium well. He powered his way through the draw here, riding that famous serve through five matches without the loss of a set.

Last year, Isner entered Miami with a 2-6 record, his long career as a staple near the bottom of the world Top 10 seemingly in jeopardy. But, at age 32, he ended up logging the best tournament win of his career, in what would become his best year on tour as well. Isner played just four tiebreakers in his march on the beach in 2018, winning three.

This year, Isner's 10-6 record entering Miami was better, but his performance in tiebreakers was worse. He ranked No. 25 in that department, winning just more than half of them (7 of 10) -- until this tournament. In Miami, he has won all nine tiebreakers he's played -- nine of his 10 sets have gone to tiebreakers -- a stat that's enough to make the hair stand up on the back of any opponent's neck. The power of that Isner serve is the main reason the 6-foot-10 veteran is so adept at winning tiebreakers.

"He will not miss many serves," Federer predicted.

Federer and Isner last met in a sanctioned tournament in 2015 in the fourth round of the Paris Masters. Isner won in three sets, sweeping two tiebreakers. That memory might give pause to a competitor with a weak stomach, but not one as enamored of the game in all its dimensions as Federer.

"I love watching the big servers," Federer told ESPN analyst Brad Gilbert after his 6-2, 6-4 defeat of another Canadian teen, Denis Shapovalov, in the semifinals. "It's exciting, like a shootout in a soccer game. I guess tomorrow I'll be playing the goalie."

It's an appropriate analogy, because the odds are nearly impossibly stacked against a goalie facing a penalty kick, which is a lot like facing a serve. Both the kick and the serve start from a standstill, a great advantage to the server. And, if accurate and powerful enough, both are nearly impossible to stop. But goalies guess, and so do returners in tennis. They also look for tells. So they sometimes end up making heroic saves -- or beating flame-throwing servers.

Entering Sunday's finals, John Isner has won all 10 of his sets -- nine by tiebreaker -- at the Miami Open thanks in large part to his dominant serve. Steve Mitchell/USA TODAY Sports

So how does Federer approach returning Isner's serve?

"Sometimes you go with momentum, sometimes with feel, sometimes you guess a little bit and sometimes you actually see it. It's a combination of things," Federer said. "Some days you feel it better than others. But the margins are slim."

Federer, who is 5-2 overall against Isner, added, "It's tough, you always just hope the stars align. You pick the right side, he picks the wrong side. Maybe he misses a serve and you try to put him in uncomfortable situations time and again, and at the end of it you somehow find a way. But he has a serve you basically cannot read. It's that simple."

To be fair, there are other components in Isner's mastery of the art of the tiebreaker. He goes into those anxiety-laden occasions with the right mindset. "I'm just finding myself pretty calm in those situations," said Isner, who for the first time in his career has reached consecutive finals in a single Masters tournament. "Maybe it's a little bit mental. I'm definitely playing my best tennis when I have been in that situation this week, there is no doubt."

The key to avoiding tiebreakers is service breaks, and earning them against Isner is somewhat counterintuitive. The first imperative is to hold serve, which puts a little pressure on an opponent. Then it's a matter of taking advantage of any little lapse or miscue of the kind to which Isner -- and every other player -- is prone. Isner reached his first Masters final at Indian Wells in 2012 only to lose to Federer, who won a tiebreaker in the first set and avoided having to play any more of them by dropping just one point on serve in the second set.

Miami Open Schedule The Miami Open runs through March 31. Here's the remaining schedule and how you can watch on the ESPN family of networks: Date Event Time (ET) Channel March 30 Women's finals 1 p.m. ESPN2 March 31 Men's finals 1 p.m. ESPN2

Another area of concern for Federer: Isner's game has taken on greater depth and richer textures since he struck up a partnership with coach David Macpherson early last year. Isner is in excellent shape physically, and he's returning better and staying in rallies longer. Just as important, he isn't as anxious about his results and stats as he once was. He attributes the change largely to the influences of Macpherson and Isner's family life -- he became a husband in December 2017 and a father last fall.

"If this was eight years ago, I might have felt that pressure [of being defending champ]," Isner said. "But now, I don't ever think about defending [rankings] points or anything like that. I used to worry about that, but I don't anymore."

This new edition of Isner will feel at liberty to swing freely and make good use of his improved return and greater ability to rally, while still knowing when to take a chance and make a placement. Shot selection will be especially critical against Federer, who, match after match, has turned this tournament into a demonstration of his own boundless skills.

"He has every shot mastered," Shapovalov said after his semifinal loss to Federer. "When he needs a shot, he's going to make it. He's going to go for it, and he's going to make it. I don't think he has any weaknesses. That's why he's the GOAT."

Federer may need his most spectacular shots to neutralize the impact of Isner's serve. On Thursday evening in the quarterfinals, Federer was able to bamboozle No. 6 seed Kevin Anderson -- a 6-foot-8 power server -- with one of his less conspicuous tools: the chip return off either wing. If Isner doesn't have his first serve dialed in, or if those stars are aligned the way Federer hopes, Federer could do damage to Isner's serve with it.

The Miami Open finals likely will come down to how well Federer defends his serve and how many chances Isner gives him to initiate play with his return. Federer will be a heavy favorite. But if you buy into his analogy about the goalie and the kick from the penalty spot, you know it may not be as simple as the math -- even Isner's math -- suggests.