Sisters Venus and Serena Williams raise their arms after their center court match at the Australian Open Tennis Championships in Melbourne, Australia on Jan. 21, 1998.
This was their first professional face-off. Venus won the match.
Venus consoles Serena after winning against her in the women's singles semifinal on the Centre Court at Wimbledon on July 6, 2000.
Serena, right, congratulates Venus after Venus defeated her to win the women's final of the U.S. Open on Sept. 8, 2001 in Flushing Meadows, New York.
Venus congratulates Serena after her victory in the final at the French Open tennis championships at the Roland Garros stadium in Paris on June 8, 2002.
Venus puts her arm around Serena in congratulations after Serena won the women's singles championship match on the Centre Court at Wimbledon on July 6, 2002.
Serena holds the U.S. Open trophy after beating her sister Venus in the women's final in Flushing, New York on Sept. 7, 2002.
Serena Williams blows kisses to the crowd after defeating her sister Venus in the women's final at the Australian Open in Melbourne on Jan. 25, 2003.
Serena holds the winning trophy as she looks over at Venus, after defeating her in the women's singles final on the Centre Court at Wimbledon on July 5, 2003.
Venus walks away after winning a point as Serena reacts in the background during their match at the U.S. Open tennis tournament in New York on Sept. 4, 2005. Venus won the match.
Venus, bottom, plays in the women's singles final against Serena on the Centre Court at Wimbledon on July 5, 2008. Venus came out victorious.
Serena greets Venus at the net after defeating her in their quarter-final match at the U.S. Open tennis tournament at Flushing Meadows in New York on Sept. 3, 2008.
Venus congratulates Serena, who was victorious during the women's singles final match of the Wimbledon Lawn Tennis Championships on July 4, 2009 in London.
Serena embraces Venus after winning the women' singles match at the Wimbledon Tennis Championships in London on July 6, 2015.
Serena, right, serves to Venus during their quarterfinals match on the ninth day of the 2015 U.S. Open Tennis Championship in Flushing Meadows, New York on Sept. 8, 2015.
Serena Williams embraces her sister, Venus, as she celebrates after winning the women's singles final at the Australian Open tennis championships in Melbourne, Australia on Jan. 28, 2017.