Oldest person ranked world tennis number one (female)

Serena Williams (USA, b. 26 September 1981) was 35 years 124 days old when she won her 23rd Grand Slam singles title, at the Australian Open in Melbourne on 28 January 2017. In doing so, the oldest Grand Slam singles winner (female) returned to the top of the women's world rankings ahead of Germany's Angelique Kerber, who crashed out of the 2017 Australian Open in Round 4.

Serena Williams first went to the top of the rankings on 8 July 2002. As of 30 January 2017 (the date the WTA updated their rankings list), she had spent a total of 310 weeks as world number one. Only Steffi Graf (Germany, 377 weeks) and Martina Navratilova (USA, b. Czechoslovakia, 332 weeks) have spent more weeks at number one.

Serena had accumulated 7,780 ranking points as of 31 January 2017, ahead of Kerber on 7,115 points.

At the start of the 2017 Australian Open, Serena Williams was tied with Steffi Graf (Germany) for the most Grand Slam singles titles won in the open era (since 1968) – 22. By winning the Australian Open, Serena is now on her own as the most decorated singles player in the open era (male or female), and is just one tournament victory away from equalling the all-time record set by Margaret Court (Australia), who won 24 Grand Slam titles in 1960–73.

Serena won the US Open in 1999, 2002, 2008 and 2012–14; the French Open in 2002, 2013 and 2015; Wimbledon in 2002–03, 2009–10, 2012 and 2015–16; and the Australian Open in 2003, 2005, 2007, 2009–10, 2015 and 2017. She is the only player (male or female) to win at least six singles titles at three of the four Slams, and the only player to claim seven titles at two of tennis' four major tournaments. She is also the only player to win at least 10 Slams in two different decades, and her tally of seven titles in Australia cannot be matched in the open era (male or female). Margaret Court won her home Slam 11 times (1960–73), but only four of them came in the open era (1969–71 and 1973), i.e., when professional players were allowed to compete in tournaments alongside amateurs.

Serena, seeded 2, beat older sister Venus Williams, 6–4, 6–4, in the Australian Open final to regain the world number one ranking from Angelique Kerber, thus making her the oldest female player to hold the No.1 spot.