Injury flare-up grounds Serena Williams

She’s the undisputed world No.1 with 21 grand slam singles titles to her name, but are cracks finally starting to appear for Serena Williams?

The 34-year-old American was forced to pull out of her 2016 season-opening Hopman Cup tie against Ukraine on Monday after succumbing to inflammation in her left knee.

Knee issues forced Williams to rest for almost two months after being knocked out in the semi-finals of the US Open in September during a dominant 2015 season.

In November, her coach Patrick Mouratoglou revealed Williams was dealing with similar knee problems to those that have hampered Rafael Nadal’s career in recent years.

“It’s just playing for so many years, the cartilage is (almost) gone,” Mouratoglou said at the time.

“Not all of it, but a big part, so the bones just hit themselves.

“She has bone bruises, and if you keep on playing with this for too long, too much, the next step is a stress fracture.”

Williams trained at Perth Arena early on Monday in the hope her knee would feel good enough to take to the court later that morning.

Although it felt better than the previous day, Williams made the decision to withdraw from the Americans’ opening tie, while declaring confidence she would be able to play against the Australia Gold team of Lleyton Hewitt and Jarmila Wolfe on Tuesday night.

At full fitness, Williams remains head and shoulders above even her closest rivals.

But if her knee problems persist, the likes of world No.2 Simona Halep, Maria Sharapova, Garbine Muguruza, Petra Kvitova, and Agnieszka Radwanska could sniff the chance to steal grand slam glory in 2016.

Halep predicts it will be an interesting year as the chasing pack attempt to knock Williams off her perch.

“Yeah, I feel that it’s a good timing to play against her,” Halep said.

“Still, I feel that she’s the best player in the world.

“I believe also that many players from top 10 can beat her.

“Many players are playing really good tennis in this moment.”

Williams won the Australian Open, French Open, and Wimbledon in 2015, with her semi-final exit at the US Open leaving her just short of a calendar grand slam.

The American needs just one more grand slam title to match Steffi Graf’s Open era record of 22.

In the absence of Williams, America crashed to a Hopman Cup loss to Ukraine on Monday, with the tournament favourites losing in both the men’s and women’s singles rubbers.

Svitolina defeated late fill-in Vicky Duval 6-4 6-1, before Alexandr Dolgopolov beat Jack Sock 6-4 6-2.

Duval, a 20-year-old American, was lured to the Hopman Cup to fulfil the role as an injury replacement in the event anyone pulled out.

Tournament director Paul Kilderry dismissed suggestions Williams was always going to miss her opening match.

“Vicky was here as a reserve for the whole tournament. It just happened that an American player got injured and she was able to fill in today,” Kilderry said.

“It’s really lucky for us.

“Every year we bring a player here as a reserve, a male and a female, and Vicky just happened to be the player this year.”

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