Four-time Grand Slam singles champion Naomi Osaka retired from her opening match against Kaia Kanepi at the National Bank Open in Toronto on Tuesday because of a back injury. The Japanese former world No. 1 and current world No. 39 was trailing 7-6(4), 3-0 when she withdrew, handing 31st-ranked Kanepi of Estonia a walkover entry into the second round at the WTA 1000 tournament, also known as the Canadian Open. “I felt my back from the start of the match, and despite trying my best to push through it I just wasn’t able to today,” Osaka said. “I’d like to pay credit to Kaia for playing well and …