Djokovic would walk off the court weeping, too. For one man, a victory over the world's No. 1 tennis player that could resurrect a career he feared was over. For the other, a stunning loss that leaves him waiting four more long years for a chance at an Olympic singles gold medal.
Del Potro, the 2009 U.S. Open champion stalked by injuries ever since, won 7-6 (4), 7-6 (2) in the first round Sunday at the Rio de Janeiro Games. After three left wrist surgeries that had him contemplating retirement, he is ranked just 145th.
But when the big Argentine is healthy and smacking his signature forehand around the court, his play is worthy of the gold-medal match -- no matter that his backhand is still a work in progress.
"Amazing, amazing match for me," del Potro said. "I didn't expect to beat Novak tonight, because I know my situation."
These two faced off for bronze in London four years ago, with del Potro winning. But when he arrived at Wimbledon in June, he hadn't appeared at a major in 2 1/2 years.
He would upset fourth-seeded Stan Wawrinka in the second round there, and even though a fatigued del Potro lost his next match, it was a hint that he might yet still have some greatness in him.
Del Potro was just 20 when he rallied from a two-sets-to-one deficit to stun five-time defending champion Roger Federer in the U.S. Open final. With the way his forehand rocked Federer that day, more Grand Slam titles seemed certain for the young Argentine.
Then came a string of wrist injuries, first to the right, then the left -- which the righty needs for his backhand. He was still slicing often Sunday, but the two-handed shot is beginning to look more comfortable. And with the way he was booming his serves and forehand, del Potro put the pressure on Djokovic from the start.
In the second-set tiebreaker, del Petro crushed two straight forehand winners on Djokovic's serve to take a 3-0 lead. He made it 5-0 before Djokovic scored a point.
Del Potro hit 41 winners Sunday, 29 on his forehand. It would be a forehand off the net cord, however, that clinched victory on his second match point.
Djokovic, a 12-time major champion, had more unforced errors than winners: 32-26. The Serb, who will be 33 at the 2020 Olympics, called this "one of the toughest losses in my life."
He's still alive in doubles, though, so a gold medal in Rio remains a possibility.
At 2-2 in the first-set tiebreaker, del Potro won three straight points to take control. A big forehand that Djokovic couldn't get back gave him three set points, and he converted the second when Djokovic's forehand sailed wide.
"In decisive moments, he just came up with some extraordinary tennis," Djokovic said of the player he described as "a good friend of mine."
Del Potro's Sunday did not start off so auspiciously -- he was stuck in an elevator at the Olympic Village for 40 minutes before Argentine handball players freed him.
"It's very difficult because del Potro has a very important match," one of his rescuers, Gonzalo Carou, said later.
A hugely difficult match from both del Potro and Djokovic's perspective -- each had the misfortune of drawing the other in the first round.
It was a raucous atmosphere in the last match of the night on center court, where Argentine fans chanted for del Potro and Brazilians would try to drown out their rivals with cheers for Djokovic.
Djokovic started the year with talk of a Grand Slam after he won the Australian Open and French Open. But he was upset in the third round at Wimbledon by 41st-ranked Sam Querrey. The 2008 Olympic bronze medalist seemed to bounce back by winning a hard-court tuneup in Toronto before coming to Rio de Janeiro.
"He told me really kind words at the end," del Potro said, "and I appreciated that."