-- Over an unprecedented career, Roger Federer has inspired countless bouts of fear and awe. But more than six years ago in the US Open final, facing an unflinching 20-year-old Argentine, Federer became unstrung.
The epic weapon that did the damage: the fierce forehand of Juan Martin del Potro that sometimes exceeded 100 mph and drew audible, collective gasps from the New York crowd.
Del Potro beat Federer in five sets for his first Grand Slam singles title, becoming, at 6-foot-6, the tallest Grand Slam champion of the Open era. He was the first player to beat Federer and Rafael Nadal in a major, and, considering his age and stage, it was quite possible to envision del Potro as their peer -- or something more -- for the foreseeable future.
There were a few occasions when the sheer heft and the pace of that wicked shot seemed beyond human limits. In retrospect, perhaps it was.
Less than eight months later, Del Potro had surgery at the Mayo Clinic in Minnesota for a split extensor carpi ulnaris tendon in his right wrist. Ultimately, the small piece of fibrous collagen tissue that connects bone to muscle couldn't withstand the repeated force of racket striking ball.
It was the first of four wrist surgeries -- the next three would be on his left wrist, used in his two-handed backhand -- in just over five years. No one, including the 27-year-old del Potro himself, knows whether he is capable of again producing the spectacular tennis he did in the fall of 2009. After enduring a maddening purgatory of surgical repair, rehabilitation, rest and repeat, del Potro ponders his first official ATP World Tour match in nearly 11 months, and only the fifth in almost two years. His current ATP World Tour ranking: No. 1,042.
Tuesday (8 p.m. ET) in southeast Florida, he'll play 23-year-old American Denis Kudla in the first round of the Delray Beach Open. In terms of tennis, it's a match with minor stakes. Yet the anxiety level promises to be excruciating -- not just in del Potro's mind, but in those of his many fans around the world.
Can that tiny tendon hold? Will the chronic ache return?
"The goal is to play the first match and get back to the hotel thinking I can play again tomorrow," del Potro told the Argentine newspaper La Nacion on Monday. "The results of the match are going to be inconsequential for me. Just being able to play in a court where I have a lot of good memories and friends coming out to see me; to feel that thrill again after being injured is a victory in itself.
"When the day comes where my wrist continues to hold up, then we'll focus more on the results, but for now it's just good to be back."
Feeling his pain
Tennis is a notoriously punishing one-on-one sport, demanding both physical and mental agility. In the vast downtime between matches, it's the mental toughness that can carry an athlete to greatness. Confidence, born from consistent results, is what separates the good from the great.
For nearly six years, in the wake of his greatest achievement, the humble product of Tandil, Argentina, has lacked that essential state of being: belief.
"I think that's the worst -- not knowing when and where or being able to schedule a tournament trip," del Potro told La Nacion. "Not knowing if I'm going to be two weeks in Buenos Aires having to go to the doctor. It's a horrible situation, but my family and friends more than anything have kept motivating me to keep going forward."
Over the past week, ESPN.com spoke with three heavily decorated players who know del Potro well and, moreover, understand the difficult place in which he finds himself.
"It can definitely mess you up, mentally," said Haas, twice the ATP's Comeback Player of the Year, from his home in Bradenton, Florida. "You don't feel like training. It's tough to stay physically strong. Obviously, it must have been very, very frustrating."
There are only seven active male players with Grand Slam singles titles. After Novak Djokovic, Federer, Nadal, Andy Murray and Stan Wawrinka, it's Cilic and del Potro, with one each. Cilic, who won the 2014 US Open champion, beat del Potro in the fourth round of the 2010 Australian Open -- del Potro's last match before undergoing surgery for the first time. The two played doubles in 2014 at Indian Wells and were also in the 2015 doubles draw there, but Cilic's shoulder injury prevented them from taking the court.
"It definitely makes it even more hard when you have been at the top of the sport," Cilic said from Marseille, France. "To be sitting on the side, waiting for his recovery is very, very difficult. He was competitive in almost every tournament he played, and he's at the age, 26, 27, when there are a lot of expectations, a time when you want to get everything out of your body.
"It's a big stage of his career to be missing. He still has the potential to come back. I hope his love for the game will drive him."
Fish, who struggled with physical and mental issues before retiring after last year's US Open, has played doubles with del Potro, and the two occasionally text. For the record, Fish has also undergone similar wrist surgery -- twice.
"It was difficult to get that right," said Fish, who played the Champions Tour event in Delray Beach, of the surgery. "Obviously, he's unlucky to have to do the same thing all over again on the other wrist. It's the same process, same timeline.
"Every athlete wants to end their career on their own terms, and Juan is no different. He's a great champion and a great friend, and I'm happy to see him back testing his wrist."
Haas, one of only two men to win the Comeback Award twice (Sergi Bruguera is the other), overcame rotator cuff surgery in 2002 and hip and elbow surgery in both 2010 and 2011.
The first time, I was 24, going on 25 and very eager to come back," Haas said. "The second time, it was more of a question. What kind of mentality do you have? What are your goals? How does the injury feel? Can you trust it?
"In the end, am I eager to do this again?"
Five years ago, del Potro raced through the field at Delray Beach, beating this year's top seed, Kevin Anderson, in the quarterfinals, Fish in the semifinals and Janko Tipsarevic in the final. It was del Potro's first title since the 2009 US Open, and it moved him from No. 166 in the rankings to No. 89.
Del Potro was asked Monday to grade his wrist on a scale of 1 to 10.
"I'm healthy enough to be able to play with other professionals to play at one hour in the morning and one hour in the afternoon; and be able to play plenty of minutes with them," del Potro said. "Delray Beach gave me the opportunity, and I'm really grateful to play a match and test out my wrist and see how it feels and what I need to keep working on.
"It's a good test to figure what my next steps are. I'm the first one that wants this to end and forget about my wrist and no longer talk about my injury. But my match isn't against my rival, but versus all these situations and myself."
Patience is a virtue
When Argentina's Davis Cup team won in the first round last March over Brazil in Buenos Aires, del Potro was there, supporting his teammates.
"He shared a lot of emotions with us," said Daniel Orsanic, Argentina's Davis Cup captain. "He put a lot of energy in his training sessions, but unfortunately, he could not be ready for the second tie [the quarterfinals against Serbia]."
Orsanic, like most of Argentina, will be watching Tuesday night.
"He'll be pretty anxious," Orsanic said. "It has been his ability to look forward that has put him in this position. But if you're at home watching, it's not the same adrenaline as playing on the tour."
Haas played del Potro five times, most recently in 2013, in Indian Wells and Washington -- and lost all five in straight sets. The forehand, he said, is still burned into his brain.
"Reminds me of Pete Sampras, that great crosscourt forehand," said Haas, 37, who has a shoulder injury. "It's shooting like an arrow, a rocket. By the time you see it, it's past you. There's nothing you can do about it.
"It was so annoying. Trust me, I'd love to see him hitting it again."
All three friends stress patience as the critical component for del Potro, who has clearly come back too quickly twice already.
"He's already taken care of the fitness," Cilic said. "Maybe play a little bit of [one-handed] tennis without the left wrist. The most important thing is to see the big picture and work your way back to being healthy for a full season. The one thing you see in athletes in any kind of sport after long breaks is they come back and suffer injuries in other parts of their bodies when they don't adjust to the intensity."
Fish said del Potro is a favorite in the locker room because of his easygoing manner, generosity and dignity.
"He's just an awesome guy, very easy to root for," Fish said. "If he never played another tennis match, he'd still have one of the best careers of our generation.
"I understand what he's been through. Let's cross our fingers and hope that everything holds up OK."
A healthy del Potro, Fish said, might even be able to step up and push the No. 1-ranked Djokovic, who has won 10 of the past 21 Grand Slam singles titles.
"Juan Martin has the type of game that he can hit through players," Fish said. "Novak is dominating the game right now. But if there was a guy out there that was healthy and who could challenge him, certainly he'd be one of them."
If he stays healthy, Haas said, "I could see him being dangerous again at the US Open." Del Potro, of course, isn't thinking about revisiting the scene of his greatest triumph. Not even close. When he announced his third surgery back in January 2015, he said his goal was to get back to the game he loved -- and no longer hate tennis.
"A lot of people identify with the problems I've had and trying to overcome them," del Potro said. "Lots of fans have sent me get-well wishes, and I'm just fighting to get back to a competitive level. And the results are meaningless compared to this battle I have with my injury.
"While it's a long road ahead, I'm giving it my all. Sooner or later, I'm going to win this fight."