Naomi Osaka’s breakthrough season came to a disappointing end on Friday when a leg injury forced her to quit her final group game of the WTA Finals in Singapore. The US Open champion was 5-3 down to Kiki Bertens in the first set at the Singapore Indoor Stadium when she called for a medical time out for treatment on her left leg and lower back. Osaka’s leg had been bothering her during her second match against Angelique Kerber, which the Wimbledon champion edged after two hours and 29 minutes. Osaka with an MTO down 3-5 in the 1st against Bertens. Her leg is hurting, as it had been in her last match as well . Here’s what she said to @BigSascha on the last changeover: pic.twitter.com/XyOZORVvy2 — WTA Insider (@WTA_insider) October 26, 2018 <!--//--><![CDATA[// ><!--\n\n\n//--><!]]> The 21-year-old also withdrew from the Prudential Hong Kong Tennis Open earlier this month because of a back injury she sustained before the China Open in Beijing. Osaka’s coach Sascha Bajin came onto court before her medical timeout out to speak to her, and asked if her leg was too painful for her to continue. WTA Finals Singapore 2018: Angelique Kerber outlasts Naomi Osaka in instant classic as Sloane Stephens edges Kiki Bertens WTA Finals Singapore 2018: Caroline Wozniacki reveals rheumatoid arthritis condition after Elina Svitolina loss “I can’t look inside you,” Bajin said. “I don’t want you to get injured. I know you’re a fighter.” Dutchwoman Bertens is now through to the semi-finals in Singapore, and the winner of Sloane Stephens against Angelique Kerber later on Friday will join her from the Red Group.