The man in question is Richard Williams who helped Venus Williams and Serena Williams achieve tremendous things in tennis becoming the most successful siblings in tennis history. That record will probably never be broken and even if we have good siblings in tennis again the iconic careers of the Williams sisters will hardly be replicated.
In the scene in the film, Venus Williams is shown taking on Sanchez Vicario and it's a match Williams remembers clearly. Recalling the match and the scene in the film she said:
"I mean, I had a huge lead. She took a bathroom break and I fell apart. I just remember distinctly thinking, ‘Gosh, I should have won that match. I shouldn't be going home.’ And I knew I had to improve myself. I think that's what I took out of it. It wasn't anything I ever said to anyone, but inside, I knew that I should have won. So I guess I was hard on myself at the time.”
Obviously, Williams came back much stronger and carved out an iconic career for herself becoming a legend of the game.