Serena Jameka Williams (born September 26, 1981) is an American professional tennis player. The Women's Tennis Association (WTA) has ranked her world No. 1 in singles on seven occasions. She became the world No. 1 for the first time on July 8, 2002, and achieved this ranking for the seventh time on January 30, 2017. On the sixth occasion, she held the ranking for 186 consecutive weeks, tying the record set by Steffi Graf for the most consecutive weeks as world No. 1 by a female tennis player. In total, she has been world No. 1 for 315 weeks, which ranks her 3rd in the Open Era among female tennis players. Williams' accomplishments and success in professional tennis have led some commentators, players, and sports writers to regard her as the greatest female tennis player of all time.

Williams holds the most major titles in singles, doubles, and mixed doubles combined amongst active players. Her record of 39 major titles puts her third on the all-time list and second in the open era: 23 in singles, 14 in women's doubles, and 2 in mixed doubles. She is the most recent female player to have held all four major singles titles simultaneously (2002–03 and 2014–15) and the third player to achieve this record twice after Rod Laver and Steffi Graf. She is also the most recent player, together with her sister Venus Williams, to have held all four Grand Slam women's doubles titles simultaneously (2009–10).

Her total of 23 Grand Slam singles titles marks the record for the most Major wins by a tennis player in the Open Era, and is second on the all-time list behind Margaret Court (24). She is the only tennis player in history (man or woman) to have won singles titles at least six times in three of the four Grand Slam tournaments, and the only player ever to have won 2 of the 4 Majors 7 times each (7 Wimbledon titles and 7 Australian Open titles). She is also the only tennis player to have won 10 Grand Slam singles titles in two separate decades. She has won an all-time record of 13 Grand Slam singles titles on hardcourt. Williams holds the Open Era record for most titles won at the Australian Open (7) and shares the Open Era record for most titles won at the US Open with Chris Evert (6). She also holds an all-time record for the most singles matches won at the Grand Slams (man or woman) with 316 matches (through the 2017 Australian Open).

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