Serena Williams has revealed that she inadvertently announced her pregnancy online while relaxing on holiday with her fiance last week.

The world number one posted a picture of herself on Snapchat with the words '20 weeks', but caused confusion by removing the post soon after during a break in Mexico.

It was later confirmed that the record-breaking 23-time grand slam champion is expecting her first child with her husband-to-be Alexis Ohanian in September.

And the iconic American admitted she did not intend to reveal that she was pregnant in such an uncustomary manner.

"Well actually it was an accident." she said. "I was on vacation just taking some time for myself.

"I have this thing where I've been checking my status and taking pictures every week to see how far along I'm going,

"I've just been saving it. I didn't really tell a lot of people to be quite honest.

"You know how social media is, you press the wrong button and... I'm like, that [having four missed calls] is weird. And I picked up and I was like, 'Oh no.' 

"But it was a good moment. I was going to wait, literally like five or six more days. But that’s okay.

"I've been tracking it, 18 [weeks], 19, every week I just take a picture and save it. I've been so good about it — this was the one time that... I slipped. But it's okay."