Michelle Obama and tennis great Billie Jean King led the tributes to Serena Williams after her U.S. Open third-round loss to Ajla Tomljanovic on Friday, likely the final match of her sparkling career. The 23-times singles Grand Slam champion's 7-5 6-7(4) 6-1 defeat sparked a flood of messages on social media, as current and former athletes across sport as well as the world at large celebrated her achievements of the last 25 years. "Congrats on an amazing career, @SerenaWilliams! How lucky were we to be able to watch a young girl from Compton grow up to become one of the greatest athletes of all time," Obama said in a tweet.