World number one Serena Williams suffered an early season injury setback Monday as she withdrew from the United States' opening Hopman Cup tie with inflammation in her knee.

Her withdrawal helped the Ukraine pairing of Elina Svitolina and Alexandr Dolgopolov record a 2-1 round robin opening win over the US in the mixed teams tournament.

Williams was replaced by American youngster Vicky Duval, on the comeback trail after being diagnosed with cancer in 2014, and the 20-year-old was no match for the world number 19.

The first set was competitive but Svitolina cruised past the tiring Duval 6-4, 6-1.

Dolgopolov then secured the tie with a comfortable 6-4, 6-2 win over a disappointing Jack Sock.

In the dead mixed doubles rubber, the US pair gained some consolation by winning 6-2, 6-3.

Williams, 34, was scheduled to play Svitolina in the women's singles to open the tie, but withdrew minutes before the start, saying she had pulled up sore after a training session earlier in the day.

The match at the mixed-teams tournament would have launched Williams's preparations for her title defence at the Australian Open in Melbourne later this month.

Williams, who has won 21 Grand Slam singles titles, sat out the tail-end of last season saying she needed "time to heal" after narrowly missing out on a rare calendar-year Grand Slam.

Her coach, Patrick Mouratoglou, conceded Williams's knees were troubling her in November.

But the player said she hoped to be fit to play in the United States' second tie against Australia Gold, one of two teams representing the hosts, on Tuesday evening.
"I'm disappointed to not take the court in Perth today," Williams said. "I had every intention to play this morning.

"Unfortunately due to inflammation in my knee I need to rest and am confident to be out there against Aussie Gold tomorrow evening."

Tournament organisers dismissed suggestions the presence of Duval in Perth as an emergency player for the event indicated they were aware of concerns over the fitness of Williams.

Williams won her sixth Australian Open last January and was on target for the coveted calendar-year Grand Slam until she suffered a shock loss to Roberta Vinci in the semi-finals at the US Open in September.

Duval only returned to tennis in August after overcoming Hodgkin's lymphoma and has a career high ranking of 87.
Source: AFP