Serena Williams grew up to become one of the best tennis players in the world. However, the 23-time Grand Slam champion's journey wasn't one without insecurities.On Wednesday, Serena joined her sister Venus Williams on Episode 2 of Stockton Street. The sisters, who together won multiple major titles, discussed several topics, one of which involved discussing Serena's early insecurities. Remembering the early tennis learning days, Serena said:"I sucked. I was bad," Serena said. "I didn’t have it. I only hit lobs. Remember, I only hit lobs. I was terrible. I mean, even when I first turned pro, I was really bad. Not that I didn’t, but I just thought I wasn’t that good. I thought I could outlast people because I was small at the time. I thought I could outlast them and just be out there. And yeah, I never really thought about it."Then, when Dad told us to pick a tournament to win so we could focus on the goal, you picked Wimbledon too. And then he said I needed to pick something different. So I picked the US Open, but I didn’t really think I would make it."The Williams sisters together won 14 women’s doubles majors between 1999–2016. A lot of credit behind their success must go to their father, Richard Williams.Serena Williams and Venus Williams' early tennis trainingSerena Williams and Venus Williams started playing tennis at an early age when they were living in Compton, California. Their father, Richard Williams, trained them for professional tennis. In the early 1990s, the family moved to West Palm Beach, Florida, so that Serena and Venus could train under Rick Macci. Even then, Richard remains deeply involved with their progress.Richard's training routine is well known. It involved the Williams sisters being asked to train early mornings (5 AM) in the dark. He would set up tough conditions, like using bad balls and broken glass at baseline to make girls run faster. Serena admitted that she hated training."I used to cut the racket strings because I didn't want to train," Serena said (via Tennis Temple). "I remember we would take a long car ride, to Anaheim or something like that. I wasn't feeling well. I took scissors and cut the strings. When we arrived, Dad took out the rackets and all the strings were broken. I remember his face at that moment; he was so sad. I felt bad, and I never did it again."While at the time, Serena didn't like training, she later acknowledged that the sisters needed every second of that training to become what they are today."But in hindsight, we needed every second of training," Serena added. "To be the best, you have to put in the time and be willing to work harder than anyone else. I think that training motivated me later on."Serena has immense appreciation for what her father did as coach, manager, protector, and vision-setter. Her career is a testament to that training. Apart from 23 major titles, she has won four Olympic Gold Medals, completed a career Grand Slam in both singles and doubles and 73 career singles titles.