The mixed doubles will decide the 20th Hopman Cup winner, after Serbia's Novak Djokovic squared the final 1-1 against the US.

Djokovic overcame shoulder trouble midway through the match to outlast and outplay Mardy Fish in a gruelling 153 minute thriller, 6-2 6-7 (4-7) 7-6 (7-4).

Earlier, Jelena Jankovic, hobbled by strained buttock and hamstring muscles for three days, forfeited her singles match against Serena Williams, giving the US a walkover win and a 1-0 lead at Perth's Burswood Dome.

Djokovic stormed to a one set lead over Fish, but then seemed to tire and go off his game, requiring on-court treatment to his shoulder before the third set.

But he regained his focus and held off Fish's power serves to take the tournament final to the mixed doubles decider.

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

"But we're fighters, so we managed to survive the last two days, we'll try to do that today."

Serbian team officials are hoping that by resting Jankovic in the women's singles against an in-form Williams, she will help Djokovic take out the first Hopman Cup title for Serbia.

Yugoslavia won the cup in 1991.

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.

Djokovic said he would be relying on Jankovic in the doubles.

"In the last couple of days her sentence before the match was, 'It's all on you'. But today it's going to be the opposite, 'Jelena, it's all on you,'."

The US is appearing in its ninth Hopman Cup final and is after its fifth title. Williams won the trophy in 2003 with James Blake.

Both the top seeded Serbs and the second seeded US team won all three of their ties in the lead up to the final.

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