Transcript for 'Backstory' behind Serena Williams' outburst over US Open 2018 controversial ump call
It was the battle that defined the 2018 U.S. Open women's final. Not one between the players on the court, but a battle between a superstar and a chair umpire. You owe me an apology! I have never cheated in my life! One centered around allegations of cheating between a player and her devoted coach. Serena Williams' meltdown would overshadow a historic match and have the world talking about tennis, and women in sports, long after the final set. Good evening, everyone, and we welcome you to the trophy presentation of the United States open. Reporter: Now ESPN's docuseries covers the darkest moment in recent tennis history. The match last year was the ugliest finish in grand slam history. It defied -- it divided friends. It divided families. People one on one side or the other, they either believed Serena and were completely against the chair umpire Carlos Ramos, or they believe Ramos and were completely against Serena. The series follows pulitzer prize-winning journalist don van netta as he peels back the layers of this event, including the lives of both Williams and Ramos, to try and understand how two found themselves at this breaking point and what it all says about tennis and society. I want to first ask you about Ramos's backstory, his history as an umpire. What is that? How would you characterize it? He was somebody who came to umpiring very early. Set it as his career goal when he was living in Portugal. Ramos was only 16 when he began working as a chair umpire. One friend said he was so focused he treated local satellite events as if they were the wimbledon final. Ramos had an incredible will to succeed. So did two little girls a world away in Compton, California. Venus and Serena Williams are perhaps the greatest sports story ever. I mean, you have two sisters, not just one Tiger Woods in the family but two in an African-American family, in a predominately white sport. This family came out of Compton and decided, "We're going to take over tennis, and we're going to win." And win she did, becoming not only the dominant player in women's tennis but one who was willing to push the envelope through her fierce will to win, challenge of the officials, to even her outfit choices. Serena won six singles titles at the U.S. Open and heading into the 2018 open she had a shot at the record books. There was an expectation that she was going to tie Margaret court's record of 24 singles titles, grand slam singles titles. And everybody expected Serena was going to win going into the match. But Williams quickly found herself down against Osaka, and whether it was the pressure of the moment or a simple slip-up, Williams and her coach would make a huge mistake. Patrick mouratoglou has been Serena's coach for seven years. Two games into the second set he believed she was in trouble, tried to help her, and he did something that would change the match. Code violation, coaching. Warning Mrs. Williams. If he gives me a thumbs up he is telling me to come on. We don't have any code, and I know you don't know that and understand why you may have thought that that was coaching, but it is not. I don't cheat to win. I'd rather lose. I am just letting you know. I was just like, what? Patrick mouratoglou is not known to be one of those coaches who coaches all the time. I didn't understand until they showed Patrick, and when they showed him it was -- I thought to go forward, to go into the net to get to the net. From that moment on the match would spiral out of control for Williams. A broken racket in frustration, another violation, and then this moment. You stole a point from me. You're a thief, too. Code violation, verbal abuse. Game penalty Mrs. Williams. Wow, game penalty. On my goodness, this is very unfortunate. Osaka would finish her off in the second set. "Backstory" follows don as he confronts Serena's coach to find out exactly what happened in that pivotal moment. It was the first time, Patrick, you ever sent a signal to Serena? Yes. And you chose to do it at that moment, why? Because I felt it was an important moment. She was -- it was probably one of the biggest moments of her career. She's in a grand slam final to equal the record of all times. And she's losing, and she's -- I feel at that moment, she's lost on the court. So I try to help her, that's my job. It was horrible for us. It was horrible for Serena. It's fantastic for tennis. It's unbelievable that was the best moment of tennis for last ten years. Tennis was everywhere. We don't have any dramas any tennis. We have dramas in all the other sports, but not in tennis. In a postmatch press conference, Serena was unbowed. The fact that I have to go through this is just an example for the next person that has emotions and that want to express themselves. And they want to be a strong woman, and they are going to be allowed to do that because of today. Serena versus the umpire pushed every button -- race, gender, power, rules, sportsmanship, fairness. But even to this day, nobody has conceded any ground. Serena Williams refuses to admit there was a clear coaching violation or that she control of her emotions. When chair umpires are at their best, they're invisible. Carlos Ramos was by the book, but he lost control of the match. Even if the backstory of Serena's fury fades with time, many will see in that singular enduring image what she wants us to see. Serena as the champion for justice, defending her honor against a man, sitting high in a chair, looking down on her. You can watch back story on Sunday.
This transcript has been automatically generated and may not be 100% accurate.