Serena Jameka Williams (born September 26, 1981) is an American professional tennis player and a former world no. 1. The Women's Tennis Association has ranked her world no. 1 in singles on five separate occasions. She became the world no. 1 for the first time on July 8, 2002 and regained this ranking for the fifth time on November 2, 2009. She is considered to be one of the greatest women's tennis players of all time.

Her 27 Grand Slam titles places her ninth on the all-time list: 13 in singles, 12 in women's doubles, and 2 in mixed doubles. She is the most recent player, male or female, to have held all four Grand Slam singles titles simultaneously and only the fifth woman in history to do so. She was also the first woman, along with sister Venus Williams, to hold all four Grand Slam doubles titles simultaneously since Martina Hingis did so in 1998. Her 13 Grand Slam singles titles is sixth on the all-time list. Williams ranks fourth in Grand Slam women's singles titles won during the open era, behind Steffi Graf (22 titles) and Chris Evert and Martina Navratilova (18 titles each). She has won more Major titles in singles, doubles, and mixed doubles than any other active player, male or female.

Williams has won two Olympic gold medals in women's doubles. She has won more career prize money than any other female athlete in history. Serena has played older sister Venus in 23 professional matches since 1998, with Serena winning 13 of these matches. They have met in eight Grand Slam finals, with Serena winning six times. Beginning with the 2002 French Open, they played each other in four consecutive Grand Slam singles finals, which was the first time in the open era that the same two players had contested four consecutive Grand Slam finals. The pair have won 12 Grand Slam doubles titles together.