Follow The Athletic’s U.S. Open coverageWelcome to the U.S. Open briefing, where The Athletic will explain the stories behind the stories on each day of the tournament.On Day 1, what it’s like inside the mind of an underdog, a rude awakening for a young champion and smooth sailing for Americans.How did an underdog keep his cool in the biggest match of his life?Across four hours and 36 minutes Sunday night at the U.S. Open, Novak Djokovic mustered just about every emotion and physical reaction there is.24-time Grand Slam champion Djokovic, 39, vomited, stumbled, winced, groaned, roared, stared and, movingly, wept his way through a five-set loss to Mariano Navone, his first defeat in the first round of a major since 2006.On the other side of the net, Navone, the 25-year-old from Argentina, was trying to take a very different approach.“For me, the mental part, I think it was the best one. I was two sets to one down, and then a break down also in the fourth, but in that moment, I feel it, you know? My game was better from there,” Navone said in a news conference.How do tennis rankings work?Ava Wallace and Lia GriffinThat doesn’t mean Navone didn’t boil over once. He showed vivid frustration with himself when he missed second-serve returns, especially when he felt he was giving Djokovic too much space to mount a victory that looked increasingly unlikely just as the 39-year-old got close to it.“I understood he wasn’t feeling very good. So it’s tough to play against him like this, because you know you have many chances to win against him,” Navone said.“it’s Novak. He’s the GOAT. He always found the key for beat someone.”Not this time. Navone, like the rest of the tennis world, had seen Djokovic win countless matches when physically compromised or appearing on the edge of oblivion. Navone knew that he had to be the one to push Djokovic over, and he did it by staying inside his head, as Djokovic showed the world everything that was going on in his.“I think my side — my best thing I did, it was thinking about my side, about my coach, about the strategy every time, the thing I had to do,” he said. EIther Stan Wawrinka, another former U.S. Open champion, or Matteo Berrettini awaits Navone in the second round.— James HansenOther notable results on Day 1: