US tennis great Serena Williams on Monday marked her fiance’s birthday and her return to the world number one ranking by posting an emotional message to her unborn baby on Instagram.

Reddit co-founder Alexis Ohanian turned 34 on Monday, the day that Williams also recaptured the women’s top ranking.

Williams’ social media post showed the 35-year-old sitting on a beach lounger in a black two-piece swimsuit that revealed her growing baby bump.

Williams is due to give birth in September, the month she turns 36 years old. In December, Williams announced her engagement to tech entrepreneur Ohanian and last week revealed she is expecting her first child and will not play again this year.

She was already expecting when she won her record-setting 23rd Grand Slam title at the Australian Open in January – where she beat her elder sister Venus in the final.

Kelly Bush Novak, Williams’ publicist, had said this week that the US star “looks forward to returning in 2018” and that was echoed by her coach Patrick Mouratoglou.

Overtaking Australia’s Margaret Court for the all-time record of 24 Grand Slam singles crowns – Williams is on 23 – is also “a determining factor” in her motivation to plough on, the Frenchman said.

Meanwhile, legendary British heptathlete Jessica Ennis-Hill has backed Williams to handle being a mum and an athlete.

Ennis-Hill took time out of athletics in 2014 and 2015 after having her first son Reggie but returned to win silver at the Rio Olympics last year. The 31-year-old is now pregnant with her second child and the 2012 Olympic heptathlon champion believes Williams will be able to juggle her responsibilities.

“It’s very individual to everyone and to what sport you do,” said Ennis-Hill. “Tennis is very different to athletics, it’s an incredible thing to do. It’s probably one of the hardest things you will face in life as a women, having a child and coming back to sport.

“I’m sure whatever she decides she’ll do a great job at it because she’s a fantastic athlete.

“Your life changes so much and you do change as a person massively. You can have benefits, when I came back and did the 800 metres I found that was a lot easier than previously – but then the other six events were harder. It’s a balancing act but you can take the good things from being an athlete to help you come back.”

*From PA and AFP