Tennis champion Serena Williams and fiancé Alexis Ohanian have welcomed their first child into the world.

The six-time US Open champion gave birth to a baby girl at a hospital near her home in Palm Beach, prompting congratulatory messages to begin circulating on social media.

Serena Williams has given birth to a baby girl. Credit:AP

Williams announced her pregnancy with Reddit co-founder Alexis Olhanian in April by posting a selfie on Snapchat with the caption "20 weeks".

The US Open was among the first to tweet the news.