Serena Williams' former coach Rick Macci recently gave his two cents on the 23-time Major winner's killer mentality. The American believes that his most successful ward was so competitive that her self-belief was never shaken even in the face of her most harrowing defeats.Macci, who helped various tennis talents rise to prominence, worked with the Williams' sisters for a short period of time at his Florida-based academy in the early 1990s. Following his tenure with them, he went on to coach future Major winners like Maria Sharapova, Andy Roddick and Jennifer Capriati.The USTPA-certified coach has always spoken about Serena Williams with high praise if his X account is anything to go by. Earlier on Sunday (July 20), the 70-year-old dissected his former client's "best quality", attributing her "beyond brutal competitive angry Pitbull" attitude to her career success."Asked the best quality of Serena. A burning rage inside this warrior that was human inferno. She was so over the top competitive in everything and that is the cornerstone," Serena Williams' former coach Rick Macci wrote on X on Sunday. "Beyond brutal competitive angry Pitbull that always felt she never lost as she just ran out of time. @serenawilliams."Rick Macci currently heads his academy in Boca Raton, Florida. The tennis veteran admittedly looks back fondly on his time with the Williams' sisters and their father, Richard.Rick Macci on Serena Williams' father Richard's dedication towards his daughters: "Every single day was about being No. 1"Last February, Rick Macci took to his X handle to express his admiration for Richard Williams, who is one of the most influential figures in recent tennis history. As documented in the 2021 biopic King Richard, he was shrewd and calculated in molding his daughters' future tennis careers.Moreover, Richard always aimed for the ceiling, as evidenced by Macci's account where he described the former as a "man on a mission"."The Number One thing I loved about the Man on a Mission, my best friend (at times) Richard Williams, was Every Single Conversation Every Single Day was about being Number One in Singles. @serenawilliams @Venuseswilliams," Macci wrote on X in February 2024.Rick Macci and Richard Williams' influence on Serena Williams and Venus Williams more than paid off as the duo won an overall 30 Major titles in singles. The Williams' sisters also occupy the top two spots in the WTA career prize money leaders' list with earnings of $94.8 and $42.6 million, respectively.