Something tells me this question has been gleaned from an American source, and one that's not particularly interested in anything that goes on anywhere outside the United States.
Althea Gibson's main claim to fame is not really that she was the first black woman to win the US tennis open. She'd already won the French Championships in 1956 (beating Britain's Angela Mortimer in the final, winning the second set by 12 games to 10) and Wimbledon in 1957. She was the first black player to win either of these tournaments. Since both Wimbledon and the French Championships are (and always have been) held earlier in the year than the US Championships, she was already the Wimbledon champion, and had been the French champion, when she became the US champion. At the risk of labouring the point, her claim to fame is that she was the first black player (man or woman) to win a Grand Slam tennis tournament; and the US Championships was the last of the three Grand Slam titles that she took. (Her best performance at the Australian Championships was in 1957, when she lost the final to her fellow American Shirley Fry.)
She went on to retain her Wimbledon and US titles in 1958. She then, reluctantly, turned professional as this was the only way she could make a decent living.
It's worth noting (IMHO) that none of the four Grand slam tennis tournaments were Open in the 1950s. The US, French and Australian championships only became known as Opens in 1968. Wimbledon, of course, still doesn't include the word Open in its official title, which is The Championships, Wimbledon.
© Macclesfield Quiz League 2018