Cincinnati - Serena Williams withdrew prior to her opening match at the WTA Cincinnati Masters on Tuesday with a back injury, another blow to the 23-time Grand Slam champion's US Open preparations.

Williams retired from the WTA final in Toronto just four games in with back spasms on Sunday, handing the title to Canadian Bianca Andreescu.

The US superstar, who trained on Tuesday morning in Cincinnati, advised officials prior to her scheduled first-round match against qualifier Zarina Diyas that she wouldn't be able to play.

"I came to Mason (Ohio) on Sunday and have tried everything to be ready to play tonight, and was still hopeful after my practice this morning," Williams said in a statement released by tournament officials.

"But unfortunately my back is still not right."

It's another injury blow for Williams, who has been hampered by knee trouble this season.

She was eyeing her first title since the 2017 Australian Open in Toronto, where the 37-year-old said she first felt back pain in her semi-final win over qualifier Marie Bouzkova.

She trailed Andreescu 3-1 when she decided she couldn't continue in Sunday's final, telling reporters the back spasms were something she's dealt with before and that they were often fleeting.

Nevertheless her decision to pull out of Cincinnati struck an ominous note with the US Open starting on August 26.

Williams, who fell to Simona Halep in the Wimbledon final in July, remains one Grand Slam title away from matching Australian Margaret Court's record of 24.

Her place in the Cincinnati draw was taken by 56th-ranked Jessica Pegula, who captured her first WTA title in Washington this month with a victory over Camila Giorgi in the final.

Results on Tuesday in the WTA/ATP Cincinnati Masters (x-denotes seeding):

Men

Second round

Novak Djokovic (SRB x1) bt Sam Querrey (USA) 7-5, 6-1

Roger Federer (SUI x3) bt Juan Ignacio Londero (ARG) 6-3, 6-4

Pablo Carreno Busta (ESP) bt John Isner (USA x13) 6-4, 6-7 (1/7), 7-6 (8/6)

First round

Denis Shapovalov (CAN) bt Joao Sousa (POR) 2-6, 6-3, 6-2

Stanislas Wawrinka (SUI) bt Grigor Dimitrov (BUL) 5-7, 6-4, 7-6 (7/4)

Andrey Rublev (RUS) bt Nikoloz Basilashvili (GEO x15) 6-7 (4/7), 6-4, 6-2

Daniil Medvedev (RUS x9) bt Kyle Edmund (GBR) 6-2, 7-5

Benoit Paire (FRA) bt Fernando Verdasco (ESP) 6-4, retired

Frances Tiafoe (USA) bt Gael Monfils (FRA) 7-6 (9/7), 6-3

Roberto Bautista Agut (ESP x11) bt Hubert Hurkacz (POL) 7-6 (8/6), 6-3

Diego Schwartzman (ARG) bt Laslo Djere (SRB) 7-6 (7/2), 6-3

Reilly Opelka (USA) bt Borna Coric (CRO x12) 4-6, 6-3, 7-5

David Goffin (BEL x16) bt Taylor Fritz (USA) 6-4, 4-6, 6-4

Women

Second round

Maria Sakkari (GRE) bt Petra Kvitova (CZE x6) 6-4, 2-6, 6-3

Venus Williams (USA) bt Kiki Bertens (NED x5) 6-3, 3-6, 7-6 (7/4)

First round

Anett Kontaveit (EST) bt Angelique Kerber (GER x13) 7-6 (9/7) 6-2

Zheng Saisai (CHN) bt Viktoria Kuzmova (SVK) 6-3, 6-1

Wang Yafan (CHN) bt Monica Puig (PUR) 3-6, 7-5, 6-3

Donna Vekic (CRO) bt Katerina Siniakova (CZE) 6-4, 6-3

Victoria Azarenka (BLR) bt Belinda Bencic (SUI x12) 6-4, 1-0 retired

Madison Keys (USA x16) bt Garbine Muguruza (ESP) 6-7 (4/7), 7-6 (7/5), 6-4

Daria Kasatkina (RUS) bt Barbora Strycova (CZE) 6-4, 7-6 (11/9)

Ekaterina Alexandrova (RUS) bt Carla Suarez Navarro (ESP) 6-4, 6-1

Hsieh Su-Wei (TPE) bt Wang Qiang (CHN x15) 6-3, 6-4

Jennifer Brady (USA) bt Ons Jabeur (TUN) 6-2, 6-2