World number one Garbine Muguruza's China Open is over at the first-round stage after she retired against Barbora Strycova.
The Spaniard has been struggling with illness which curtailed her Wuhan Open campaign last week and called time on her clash with the Czech when trailing 6-1 2-0.
Sloane Stephens is still waiting for her first win since lifting the US Open last month as the American was beaten by compatriot Christina McHale, who dropped just three games in a 6-0 6-3 success.
There was also an early exit for eighth seed Dominika Cibulkova, who was ousted 7-6 (7/4) 6-1 by Elise Mertens, and 10th seed Angelique Kerber, beaten 6-4 6-4 by Alize Cornet.
Coco Vandeweghe kept up her hopes of a place in the WTA Finals with a 6-2 6-2 success over Donna Vekic while Caroline Wozniacki made light work of Anastasia Pavlyuchenkova, winning by the same scoreline.
Shuai Peng beat Shelby Rogers 7-6 (7/3), Elena Vesnina profited from Magda Linette's retirement while Varvara Lepchenko was a 4-7 6-4 6-2 winner against Lesia Tsurenko.
Jack Sock and Kevin Anderson saw off Roger Federer and Novak Djokovic 6-7 6-3 10-6 in the Laver Cup doubles to give Team World their first point of the tournament in Chicago.
US Open champion Naomi Osaka reached the semi-finals of the Pan Pacific Open with a straight-sets victory over Barbora Strycova.
Former world number three Stan Wawrinka continued his return to form by beating defending champion Damir Dzumhur to reach the last four of the St Petersburg Open.
World number one Rafael Nadal has been forced to withdraw from the China Open in Beijing and the Shanghai Masters next month as he recovers from his latest knee problems.
Novak Djokovic claimed his second Grand Slam in succession and 14th in his illustrious career with a 6-3 7-6 6-3 victory over Juan Martin Del Potro at the US Open.
Serena Williams was left furious and in tears after a huge controversy involving a coaching violation as Naomi Osaka won the US Open final 6-2 6-4.
The final Grand Slam of the year takes centre stage from the 27th as the US Open begins in New York and bet365 will be Live Streaming the action to selected countries across the world.
Novak Djokovic breezed past Kei Nishikori 6-3 6-4 6-2 to secure his place in the final of the US Open with consummate ease.
A distraught Rafael Nadal was forced to retire through injury after trailing 7-6 6-2 as Juan Martin Del Potro progressed through to the US Open final where he will face Novak Djokovic.