PERTH • Australian Open organisers and fans are holding their collective breaths after three of the world's top four women tennis stars were hit by injury setbacks yesterday.

But Serena Williams, Maria Sharapova and Simona Halep all stressed that they were merely taking precautionary measures with less than two weeks to go till the first Grand Slam of the year.

World No. 1 Williams was forced to retire from her first singles match at the mixed-team Hopman Cup in Perth yesterday.

The 34-year-old had already missed the United States' opening tie against Ukraine owing to inflammation of her left knee, but took to the court against Australian Jarmila Wolfe. And the American retired when trailing 5-7, 1-2, having appeared extremely restricted in her movement during the match, particularly on her left side.

It appeared to cast significant doubts over her title defence in Melbourne, where she will aim to lift the Daphne Akhurst Memorial Cup for a seventh time later this month.

Williams admitted she retired because she was being hampered by the knee and was unable to move around the court as she wanted.

But the winner of 21 Grand Slam singles titles was confident it was only a minor problem and even refused to rule herself out of tomorrow's Hopman Cup tie against the Czech Republic.

"I just have some inflammation that's been going away very slowly," she explained. "It's still there, it's going away, but just needs a little more time. (It needs) a little rest, a little treatment.

"It's not even a bump, just a minor thing in the road and I'm gonna fly over it."

She said last season, which saw her on target for a calendar-year Grand Slam until she was beaten by Italian Roberta Vinci in the US Open semi-finals in early September, had taken a heavy physical toll.

She barely played after the US Open, although she returned in the International Premier Tennis League in Manila last month.

While Williams failed to complete her match yesterday, Sharapova and Halep could not even start theirs as they pulled out of the Brisbane International yesterday.

Sharapova withdrew from her season-opening event with a left forearm injury with an eye on the Jan 18-31 Australian Open.

"I hurt my forearm in practice a couple of days ago," the world No. 4 said in a statement. "This is such a special event for me, especially being the defending champion. I'm looking forward to seeing all my fans in Brisbane next year."

The Russian had been due to play compatriot Ekaterina Makarova in the first round yesterday.

Her decision not to risk her Australian Open campaign comes after the latter half of her 2015 season was wrecked by injuries, first to her right leg which forced her to miss the US Open.

She retired from her comeback match at the Wuhan Open in China in September with a left forearm injury, returning only for the WTA Tour Finals in Singapore at the end of October.

Sharapova was joined on the sidelines in Brisbane by Romanian world No. 2 Halep later yesterday, who blamed a left leg injury for her decision to pull out.

The top seed, who was due to play former world No. 1 Victoria Azarenka in the second round, said she first injured her Achilles tendon in August and had been experiencing some pain over the past two weeks.

"I thought that I was ready to play - I came here like I wanted to play, to start the year here, but I cannot have a full match, and I thought that it's more important to be ready and to take a break," she told reporters.

Halep expects to be fit for the Australian Open and intends to play in Sydney next week.

"It's nothing dangerous," she said of the injury. "I did an MRI and it's nothing dangerous, but it's still an inflammation. I have just to take a few days' rest."

Over in China, two-time Wimbledon champion Petra Kvitova bowed out of the Shenzhen Open in the first round, retiring with illness in a shock exit for the world No. 6.

The second seed was broken twice as she lost the first set 2-6 against China's world No. 72 Zheng Saisai, and dropped out before a point was played in the second set.

It was Kvitova's second consecutive first-round exit in China, after she went out at the first hurdle at the China Open in October - when she played on painkillers while suffering the ongoing effects of glandular fever.

AGENCE FRANCE-PRESSE