The United States won the Hopman Cup for the fifth time with a 2-1 victory over Serbia in the final in Perth.
Williams and Fish gave the USA their fifth win in nine finals
Mardy Fish and Serena Williams beat Novak Djokovic and Jelena Jankovic 7-6 (7-5) 6-2 in the decisive mixed doubles after the earlier singles were shared.
Williams had a walkover when Jankovic pulled out of the singles because of an ongoing thigh injury.
And Djokovic hauled Serbia level with a 6-2 6-7 (4-7) 7-6 (7-4) victory before the USA took the doubles.
Jankovic decided to forfeit her singles match and concentrate on the mixed doubles.
"She will be far better off preparing for the doubles, which is much less strenuous," said tournament doctor Hamish Osborne.
Djokovic too was struggling as Serbia played their third match in three days and he received treatment on his shoulder, which has been troubling him all week, during his singles tie.
"I wasn't really sure if I was going to be able to continue," he said. "But whenever you are on the court you want to win and you could see that today."
Williams paid tribute to her opponents, saying: "The Serbian team, they really deserve a big round of applause because they were both fighting with injury."
She added: "I had two really good matches, and had two great mixed matches.
"Seeing as I am playing singles and doubles (at the Australian Open) I am feeling really match fit."
And Fish was full of praise for his team-mate. "I had the best partner here," he said. "She has some great credentials and she played great."