In a short but realistic response, Ilana Kloss, commissioner of World Team Tennis (WTT) underlined an elemental rationale: CEOs who make the decisions on who becomes the face-image of their company are still more often white males, and better-looking females draw their attention. It would only be fair to state that Ms. Kloss has been in a relationship with former grand dame of tennis, Billie Jean King, for 30 years, and that Ms. King lost essentially all her endorsements in a heartbeat when it was revealed, in 1981, that she was being sued by a female lover for what Ms. King called 'galimony'.
Even more succinctly, TV analyst, Mary Carillo, put the same Serena-Federer situation forth in a one-to-one case study nearly 20 years ago, during a match between Martina Hingis-- who was voted one of the '30 Legends of Women's Tennis' despite a truncated career-- and media darling Anna Kournikova.
The two were frequent doubles partners, winning two Grand Slam titles together (Australia 1999, 2002), and a basic, "Whose future would you rather have?" question arose about their careers shortly after Kournikova, at 16, had gotten to the semi-finals at Wimbledon in 1997. 
Carillo's response was straight-forward: Martina had all the shots, skills, mental toughness, and she was very probably going to have an outstanding professional career. That said, her addendum was Truth-- Kournikova was an exceptionally good-looking young lady, and based on that, her paychecks in the future would probably be worth a lot. Bottom line analysis: Hingis was going to be more successful on the court ($21.8 million in career earnings), and Anna would put lots in the bank, too.
That she was forced from tennis by back problems at the tender age of 21, and was romantically linked to a long string of high profile athletes and male companions, did nothing to diminish the online popularity of Anna in bikinis. 
Given the almost undeniable historical facts of 'Like-Like More-Like Not as Much', how the endorsement world should deal with a figure like Serena Williams is not going to be accepted-resolved in any real way. Basketball star LeBron James had $90 million in endorsements at the time he joined the NBA out of high school; Tiger Woods was similarly bankable, almost iconic, before his marital situation and knee-back problems stopped his athletic success.
Add in examples like the out-spoken if undeniable greatness of Ms. King, or the less-acceptable-at-the-time sexual orientation of all-time great, Martina Navrativola, and the answer is fairly clear; Ms. Kloss isn't saying anything most of us would understand at a basic level: Talent, money, and political correctness will probably never be 100% in sync.