An upper right leg injury to Jankovic — and subsequent walkover by Williams in the opening women's singles — had given the Americans a 1-0 lead over top-seeded Serbia hours before the scheduled start of the final Friday.

Djokovic, despite receiving treatment for a sore right shoulder, then beat Fish 6-2, 6-7 (4), 7-6 (4) to keep Serbia in contention.

But the Americans' mixed doubles victory — coming back from being down 4-1 in the opening set — clinched the title in the eight-nation tournament. It was the ninth time the Americans had qualified for the final in the Hopman Cup's 20 years.

"The Serbian team, they really deserve a big round of applause because they were both fighting with injury," said Williams, who begins defense of her Australian Open singles title on Jan. 14 in Melbourne.

Williams won the Hopman Cup title for the U.S. with James Blake in 2003.

Jankovic pulled out of both her singles matches on the previous two days at the tournament, and in both cases, she came back to win the clinching mixed doubles with Djokovic — against France on Wednesday and Argentina on Thursday.

"Obviously you saw my condition today, and you're going to see us in wheelchairs in the mixed doubles," Djokovic joked.

"But we're fighters, so we managed to survive the last two days. As professional athletes, you always want to win when you are on the court, and you saw that today."

Tournament doctor Hamish Osborne said Jankovic would not be able to play two matches because she would need extended medical treatment after her singles match.

"The treatment she has been receiving is painful and she will be far better off preparing for the doubles, which is much less strenuous," Osborne said.