Juan Martin del Potro was back, or so he thought. It was February 2015, and, for the second time in the past 11 months, he had undergone surgery on his left wrist at the Mayo Clinic.

After missing the final nine months of the 2014 season, del Potro was ready to return to where he had finished 2013 – at No. 5 in the ATP Rankings.

The comeback started promising. Del Potro reached the quarterfinals in Sydney. But the pain in his left wrist remained, and in June,del Potro returned to Rochester, Minn., for a third operation on his damaged left wrist.

What followed were the darkest days of del Potro's comeback. He felt depressed and wondered if he'd play tennis again. The 2009 US Open champion contemplated quitting.

Plenty of players, including Brazil's Gustavo Kuerten and Magnus Norman of Sweden, had undergone major surgeries and never returned to the court as the same player. Would del Potro join the regretful list?

“The worst moment was in 2015 when I was close to quitting this sport because I couldn't find a way to fix my wrist problems. I had been suffering a lot. I got depressed for couple of months also. I didn't get the chance to feel better with myself, to do this again,” del Potro said.

He, of course, has returned, and is playing better than ever. On Sunday, he will meet Novak Djokovic for a chance at his second US Open title.

WATCH: 50 for 50 - Juan Martin del Potro, 2009 men's singles champion

play video 50 for 50: Juan Martin del Potro, 2009 men's singles champion

But Delpo's recovery from his fourth wrist surgery has been a winding one. He began his 2016 in February in Delray Beach ranked No. 1,042, and in his first six months, he reached only two semifinals – both at ATP 250 events. But then came the Rio Olympics, where the Argentine, No. 141 in the ATP Rankings, upset then-No. 1 Djokovic and then-No. 5 Rafael Nadal before winning the silver medal.

“[It] all kind of started with him in Rio, Olympic Games. That's where he got, I feel like, a great confidence boost. Obviously he played ever since very consistent on the big tournaments... He's playing tennis of his life, without a doubt, in the last 15 months,” Djokovic said.

Del Potro followed his Rio run by making the quarterfinals of the US Open and winning his first title in 33 months in Stockholm. He finished the season by helping Argentina win its first Davis Cup title.

“Everything that I got, it's special,” del Potro said.

Last season saw more steady progress for the Tower of Tandil, as del Potro made the semifinals at the Open for the first time since 2009 and repeated as champion in Stockholm.

WATCH: Highlights - 2017 US Open semifinal - Rafael Nadal d. Novak Djokovic

play video Nadal vs. del Potro

But he struggled against the Top 2 players in the ATP Rankings, going 0-7. Tennis observers wondered, fairly, if del Potro, who was still gaining confidence with his topspin backhand, would ever return to the very top of the game.

“I feel better with my backhand, but it's not good enough to win a title like this or to win the top guys in a row,” del Potro said last year after falling to Nadal in the US Open semifinals.

But everything – his health, his backhand and his confidence – has come together this season for the Argentine to have his best year since 2009, when he won the US Open and snapped Roger Federer's 40-match New York win streak in the final.

This year, del Potro won his maiden ATP Masters 1000 title in Indian Wells – beating Federer again – made his second Roland Garros semifinal and posted his best Wimbledon result (quarterfinal) since 2013. Del Potro is also at a career-high ranking of No. 3.

“I didn't expect to get this kind of emotions playing tennis again. Reaching finals, winning titles, having my highest ranking ever in this moment, everything is almost perfect,” Del Potro said.

On Friday afternoon, the 29-year-old avenged his 2017 semifinal loss to Nadal gained his 10th win against a world No. 1, which is the most by any player who has never reached the top spot.

Never Been No. 1 But Came Up Big Against No. 1

Player

Wins against No. 1

Juan Martin del Potro

10

Michael Chang

7

Richard Krajicek

7

Roscoe Tanner

6

David Ferrer

5

Goran Ivanisevic

5

Magnus Larsson

5

Tom Okker

5

“He's a big-time player. He's a big-match player. He's a Grand Slam winner,” Djokovic said.

Del Potro can bolster that reputation once more on Sunday. Since 2002, outside the Big Four, only Stan Wawrinka has managed to win more than one Grand Slam title. Del Potro could have his health and his second US Open title. Everything would be perfect.

“I cannot believe that I will have a chance to play another Grand Slam finals in here, which is my favorite tournament. So it would be special to me,” del Potro said. “It will be a difficult match... in the finals, anything can happen. If I win, great. If not, I been playing a great tournament and I will be happy anyways.”

play video Press Conference: Juan Martin del Potro, Semifinals