Tennis fans have grown accustomed to the sight of Juan Martin del Potro struggling to make yet another comeback.

Roger Federer, not so much.

Yet when the men square off Friday at the Miami Open, both will be trying to overcome the anxieties and hesitations created by injury and surgery.

Del Potro, now ranked No. 366, was once the greatest threat to the men perched on the highest ledge of the game. His setbacks (surgery on both wrists with absences better measured in entire seasons than in days) and the price he paid were incomparably more severe and career-shaping than Federer's.

Roger Federer has not played a match since losing to Novak Djokovic in the Australian Open semifinals. Gustavo Caballero/Getty Images

But Federer is returning from his first adventure under the knife. He underwent arthroscopic knee surgery following a freak accident that occurred at his hotel on the day after he lost to Novak Djokovic at the Australian Open semifinals.

Federer cleared up the mystery surrounding that mishap in a Miami news conference Thursday, revealing that his injury occurred while he was running a bath for his twin daughters. He said he turned on his feet, heard a "click" and felt something "strange" in his knee. By morning, his joint was badly swollen.

That was fewer than two months ago, and while it could be easy to pooh-pooh the episode, he was on crutches for just 12 days and wasn't able to resume full-out training until nine days ago.

As the No. 3-ranked Federer said following the operation, "When I woke up and looked at my knee, it was like, 'This doesn't feel like my leg. I can't believe I did the operation, and I hope it will come back from here.' That's when I got scared."

That's not a good place to be for a father-of-four tennis professional pushing 35 -- not even one named Federer.

What makes this matchup so tantalizing is that it's a rivalry interrupted. The serial injuries suffered by del Potro derailed what might have developed into a rivalry to match any that Federer would become embroiled in. Del Potro's breakdowns might have even prevented him from beating Andy Murray for fourth place in the Big Four stable. Instead, the Argentine has become the man of the perpetual comeback.

In 2008, he was a promising 19-year-old who had already cracked the top 10. A year later, he upset Federer, who was at the absolute zenith of his career, in the US Open final. Del Potro did it with a display of overwhelming power that seemed like a reality check for all those -- including Federer himself -- who had started to confuse tennis with some kind of ballet.

That following January, del Potro was up to No. 4 in the rankings but dealing with tendinitis in his right wrist. The injury was so severe that he needed surgery. He was unable to defend his US Open title and played just six matches for the year.

Del Potro fell to No. 485 in the world, but his slide was short-lived. In 2011, he had soared back to No. 11 and was named ATP Comeback Player of the Year.

But the drama was far from over. He underwent another surgery on his wrist in 2012. And then another last season, which kept del Potro out of action for 11 months before returning at Delray Beach in February.

Back when the future looked so promising for del Potro, he detonated flat drives that hit opponents like a tidal wave. His serve was a bludgeon. His backhand was a reliable weapon as well as a versatile tool for doing the deft work with an opponent approaching the net.

Throughout the years, the edge on some of del Potro's tools seem to have dulled. It's as if all the time off, all the thinking and waiting, has stifled something that once burst forth so naturally, so spontaneously. Perhaps he can reclaim it in time with wins and confidence. He's been down the comeback road before.

When del Potro and Federer meet Friday, it will look a lot like the past seen through a warped lens.

"At the end, I'm going to focus on my own game," Federer said. "We're both in a similar situation, [but] his injury was much, much greater."

That's Federer-speak for "Advantage, Roger."

This is all new to Federer, though. Minor as his setback may seem, it isn't insignificant. No calling card from mortality ever is. That left knee betrayed him in an innocuous Melbourne bathroom.

Will it fail him when he cuts right to stab a volley? He needs to know. He needs to test himself. That's why Federer is so eager to get back to the game despite the frustrations he's known in Miami since winning back-to-back titles in 2005 and 2006.

Federer has been as far as the semis in Miami just twice since then, and he skipped the event two of the past three years.

Bear in mind, the men haven't played since 2013, and while Federer won their past two meetings, del Potro is 3-2 against the Swiss dating back to 2012. Still, Federer holds a decisive 15-5 overall lead.

Their head-to-head history will likely mean little Friday. The players will be competing against not only each other but themselves.

Nobody understands the meaning of "comeback" better than Juan Martin del Potro -- not even Roger Federer.