They were once tennis’ inseparable golden girls but siblings Venus and Serena Williams’ formerly close relationship is reportedly under strain.

The tennis champions, who have won three Olympic golds together, have not been seen together off court in recent weeks and there has been speculation as to whether Serena’s constant partying is to blame.

According to the New York Post, the world number one’s late nights and recent disappointing doubles performances have irritated her older sister – who is known for her more laid-back lifestyle.

Family tension: Sisters Venus and Serena Williams’ formerly close relationship is said to be under strain

The website alleged that an insider was overheard during the US Open saying ‘They’re not in a good place right now. Venus is angry with Serena,’ because of her active social life off the court.

And on Tuesday, Venus, 34, was noticeably absent when Serena debuted her HSN clothing line at New York Fashion Week.

The 32-year-old showed she’s not only a top athlete but a budding fixture in the fashion world as she walked the runway to launch her Spring fashion line.

Party time: Serena partied with her tennis rival  and close pal Caroline Wozniacki on Monday night but there was no sign of Venus

The catwalk excursion comes just days after she won her sixth US Open over Caroline Wozniacki, who also attended the fashion show.

But while Serena has enjoyed individual success, the sisters lost in the US women’s doubles quarterfinals – just months after they had to pull out at Wimbledon when Serena complained of a mysterious illness.

The sisters, who are known to post photos of each other on social media site Instagram, have recently stopped sharing the family pictures. The last photo of Venus on Serena’s feed was in June, while Serena was last seen on Venus’ feed two months ago.

Where’s the love? The world number one’s late nights and recent disappointing doubles performances have irritated her older sister Venus, according to the New York Post

Venus’ rep, Lisa Sorensen, told the New York Post’s Page Six that Venus was not with Serena after the US Open because she went directly to Canada to prepare for the Quebec City WTA Open.

She denied any rift and said: ‘I am very surprised by these accusations. They are completely false. Venus and Serena always have been and continue to be close and supportive of one another.’

Serena’s rep, Kelly Bush Novak, also denied any bad blood between the sisters, and called the story an ‘absolute lie and reckless . . . They continue to be each other’s biggest fans and a beacon of support for each other. There is no change in their very close and special relationship.’

Party lifestyle: Serena partied with Gigi Hadid, Caroline and Joe Jonas in New York after her US Open win

Meanwhile, it is not just Serena’s relationship with her sister which is allegedly suffering thanks to her social life.

The tennis star pulled out of a number of TV interviews on Monday after spending Sunday night partying with pals including Wozniacki, Gigi Hadid and Joe Jonas.

Serena was booked to appear on NBC’s Today and ABC’s Good Morning America  and Live With Kelly and Michael with her US Open trophy but cancelled because she was ‘too tired’, according to the New York Post’s Page Six.

Sport stars: The two famous sisters at the US Open Tennis Championships in 2013