Reacting to 23-time singles' Grand Slam champion Serena Williams announcing her retirement from tennis, ex-world number one tennis player John McEnroe said Serena is at the same level as Michael Jordan and LeBron James. "She's...one of the...greatest athletes in the history of any sport," he added. "Anyone that saw...'King Richard'...realises...where she's now is unbelievable," stated McEnroe.