Serena & Venus Williams at the Olympics

Williams sisters Venus and Serena compete for gold at the All-England Club in London during the Olympics.

Tennis player Serena Williams attends the 2017 Glamour Women of the Year Awards at the Kings Theater in Brooklyn, New York.

Tennis player Serena Williams attends the 2017 Glamour Women of the Year Awards at the Kings Theater in Brooklyn, New York, U.S., November 13, 2017. REUTERS/Andrew Kelly

Tennis player Serena Williams attends the 2017 Glamour Women of the Year Awards at the Kings Theater in Brooklyn, New York.

Tennis player Serena Williams attends the 2017 Glamour Women of the Year Awards at the Kings Theater in Brooklyn, New York, U.S., November 13, 2017. REUTERS/Andrew Kelly

Tennis player Serena Williams attends the 2017 Glamour Women of the Year Awards at the Kings Theater in Brooklyn, New York.

Tennis player Serena Williams attends the 2017 Glamour Women of the Year Awards at the Kings Theater in Brooklyn, New York, U.S., November 13, 2017. REUTERS/Andrew Kelly

Tennis player Serena Williams attends the 2017 Glamour Women of the Year Awards at the Kings Theater in Brooklyn, New York.

Tennis player Serena Williams attends the 2017 Glamour Women of the Year Awards at the Kings Theater in Brooklyn, New York, U.S., November 13, 2017. REUTERS/Andrew Kelly

Tennis player Serena Williams attends the 2017 Glamour Women of the Year Awards at the Kings Theater in Brooklyn, New York.

Tennis player Serena Williams attends the 2017 Glamour Women of the Year Awards at the Kings Theater in Brooklyn, New York, U.S., November 13, 2017. REUTERS/Andrew Kelly

Tennis player Serena Williams attends the 2017 Glamour Women of the Year Awards at the Kings Theater in Brooklyn, New York.

Tennis player Serena Williams attends the 2017 Glamour Women of the Year Awards at the Kings Theater in Brooklyn, New York, U.S., November 13, 2017. REUTERS/Andrew Kelly

Tennis - Australian Open - Melbourne Park, Melbourne, Australia

Tennis - Australian Open - Melbourne Park, Melbourne, Australia - early 29/1/17 Serena Williams of the U.S. poses with the Women's singles trophy after winning her final match. REUTERS/Edgar Su

Tennis - Australian Open - Melbourne Park, Melbourne, Australia

Tennis - Australian Open - Melbourne Park, Melbourne, Australia - early 29/1/17 Serena Williams of the U.S. poses with the Women's singles trophy after winning her final match. REUTERS/Edgar Su

Tennis - Australian Open - Melbourne Park, Melbourne, Australia

Tennis - Australian Open - Melbourne Park, Melbourne, Australia - 28/1/17 Serena Williams of the U.S. serves during her Women's singles final match against Venus Williams of the U.S. .REUTERS/Edgar Su/Files

Tennis - Australian Open - Melbourne Park, Melbourne, Australia

Tennis - Australian Open - Melbourne Park, Melbourne, Australia - 17/1/17 Serena WIlliams of the U.S. serves during her Women's singles first round match against Switzerland's Belinda Bencic. REUTERS/Jason Reed

Serena Williams attends a news conference in New York

Serena Williams attends a news conference in New York March 8, 2016. REUTERS/Brendan McDermid

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates with the trophy after winning the final against USA's Venus Williams REUTERS/Edgar Su TPX IMAGES OF THE DAY

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates after winning the final against USA's Venus Williams REUTERS/Edgar Su

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates after winning the final against USA's Venus Williams REUTERS/Edgar Su

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki kisses the trophy as she celebrates winning the final against USA's Venus Williams REUTERS/Edgar Su

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates with the trophy after winning the final against USA's Venus Williams REUTERS/Edgar Su

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates winning the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates with the trophy after winning the final against USA's Venus Williams REUTERS/Edgar Su

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017. Denmark's Caroline Wozniacki celebrates during the final against USA's Venus Williams. REUTERS/Edgar Su

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates with the trophy after winning the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki stands next to the trophy after winning the final against USA's Venus Williams REUTERS/Edgar Su

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates with the trophy after winning the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates after winning the final against USA's Venus Williams REUTERS/Edgar Su

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates after winning the final against USA's Venus Williams REUTERS/Edgar Su

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki kisses her partner David Lee as she celebrates winning the final against USA's Venus Williams REUTERS/Edgar Su

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki in action during the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates winning the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates during the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 USA's Venus Williams during the final against Denmark's Caroline Wozniacki REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki in action during the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates winning the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates winning the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates winning the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates winning the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates winning the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates winning the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates with the trophy after winning the final against USA's Venus Williams REUTERS/Jeremy Lee

WTA Tour Finals

Tennis - WTA Tour Finals - Singapore Indoor Stadium, Singapore - October 29, 2017 Denmark's Caroline Wozniacki celebrates with the trophy after winning the final against USA's Venus Williams REUTERS/Jeremy Lee