By winning the 2009 Australian Open, her 10th Grand Slam singles title, Serena joins the following (4) Tennis Greats who have won 10 or more Grand Slam Singles Titles (in the Open Era): Steffi Graff (22), Chris Evert (18), Martina Navratilova (18) and Margaret Smith Court (11). Serena passes Monica Seles and Billie Jean King (who won 9 each in the Open Era).
Just in case you are wondering about the term "Open Era," in 1968, commercial pressures led to the abandonment of the distinction between professionals and amateurs, inaugurating the Open era, in which all players could compete in all tournaments, and top players were able to make their living from tennis. Thus, the open era in tennis began in 1968, when the Grand Slam tournaments, such as Wimbledon, abandoned the longstanding rules of amateurism and allowed professionals to compete. The first Grand Slam tournament to go "open" was the French Open (Roland Garros).