Roger Federer, of Switzerland, returns a shot from Juan Martin del Potro, of Argentina, during a tennis match at the Miami Open, Monday, March 27, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)
A tennis fan takes a photo during player introductions before a tennis match between Roger Federer, of Switzerland, and Juan Martin del Potro, of Argentina, at the Miami Open, Monday, March 27, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)
Caroline Wozniacki, of Denmark, returns a shot from Garbine Muguruza, of Spain, during a tennis match at the Miami Open, Monday, March 27, 2017, in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)
Garbine Muguruza, of Spain, leaves the court after she retired from a tennis match against Caroline Wozniacki, of Denmark, at the Miami Open, Monday, March 27, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)
Venus Williams hits a return to Patricia Maria Tig during the Miami Open tennis tournament, Sunday, March 26, 2017, in Key Biscayne, Fla. (AP Photo/Lynne Sladky)
Rafael Nadal, of Spain, serves the ball to Philipp Kohlschreiber, of Germany, during the Miami Open tennis tournament, Sunday, March 26, 2017, in Key Biscayne, Fla. (AP Photo/Luis M. Alvarez)
Philipp Kohlschreiber, of Germany, returns the ball to Rafael Nadal, of Spain, during the Miami Open tennis tournament, Sunday, March 26, 2017, in Key Biscayne, Fla. (AP Photo/Luis M. Alvarez)
Rafael Nadal, of Spain, returns the ball to Philipp Kohlschreiber, of Germany, during the Miami Open tennis tournament, Sunday, March 26, 2017, in Key Biscayne, Fla. (AP Photo/Luis M. Alvarez)
Svetlana Kuznetsova, of Russia, returns a shot from Taylor Townsend during a tennis match at the Miami Open, Sunday, March 26, 2017, in Key Biscayne, Fla. (AP Photo/Wilfredo Lee
Angelique Kerber, of Germany, waves to the crowd after defeating Shelby Rogers, of the United States, 6-4, 7-5 during the Miami Open tennis tournament, Sunday, March 26, 2017, in Key Biscayne, Fla. (AP Photo/Lynne Sladky)
Angelique Kerber, of Germany, hits a return to Shelby Rogers during the Miami Open tennis tournament, Sunday, March 26, 2017, in Key Biscayne, Fla. (AP Photo/Lynne Sladky)
Milos Raonic, of Canada, looks up at the sky during a brief sprinkle at the start of a tennis match against Viktor Troicki, of Serbia, at the Miami Open, Friday, March 24, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)
Milos Raonic, of Canada, returns a shot during a tennis match against Viktor Troicki, of Serbia, at the Miami Open, Friday, March 24, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)
Stan Wawrinka, of Switzerland, returns a shot from Horacio Zeballos, of Argentina, during a tennis match at the Miami Open, Saturday, March 25, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)
Frances Tiafoe returns a shot from Roger Federer, of Switzerland, during a tennis match at the Miami Open, Saturday, March 25, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)
Roger Federer, of Switzerland, tosses the ball to serve to Frances Tiafoe during a tennis match at the Miami Open, Saturday, March 25, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)
Venus Williams, of the United States, follows through on a return to Beatriz Haddad Maia, of Brazil, during the Miami Open tennis tournament, Friday, March 24, 2017, in Key Biscayne, Fla. (AP Photo/Luis M. Alvarez)
Rafael Nadal, of Spain, returns the ball to Dudi Sela, of Israel, during the Miami Open tennis tournament, Friday, March 24, 2017, in Key Biscayne, Fla. (AP Photo/Luis M. Alvarez)
Rafael Nadal, of Spain, gestures after defeating Dudi Sela, of Israel, during the Miami Open tennis tournament, Friday, March 24, 2017, in Key Biscayne, Fla. (AP Photo/Luis M. Alvarez)
Ajla Tomljanovic, of Croatia, celebrates after defeating Elena Vesnina, of Russia, 3-6, 6-4, 7-5, in a tennis match at the Miami Open, Friday, March 24, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)