TORONTO (AP) -- Naomi Osaka's recent struggles continued Tuesday with an early exit at the National Bank Open.
The four-time Grand Slam champion retired from her first-round match with a back injury. She was trailing 7-6 (4), 3-0 against Estonia’s Kaia Kanepi when she withdrew from the contest.
“I felt my back from the start of the match, and despite trying 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 want to wish her all the best for the rest of the tournament.”
Entering the tournament, Osaka had been eliminated from her last three competitions in the first or second round, including a straight-sets loss to Coco Gauff at last week’s Mubadala Silicon Valley Classic. That was her first tournament since the French Open as she recovered from an Achilles injury.
Prior to that run, she had her best tournament of the year making it to the final of the Miami Open in early April before losing to top-ranked Iga Swiatek.
The 31st-ranked Kanepi will next play No. 8 Garbine Muguruza of Spain.
In other matches, 10th seeded Gauff beat fellow American Madison Brenglem 6-1, 6-3; China’s Zheng Qinwen got past Canada’s Rebecca Marino, 3-6, 7-6 (5), 6-4; Italy’s Camila Giorgi defeated ninth-seeded Emma Raducanu of England, 7-6 (0), 6-2. Elise Mertens of Belgium, Sara Sorribes Tormo of Spain, Ajla Tomljanovic of Russia, Amanda Anisimova of the United States, and Zhang Shuai of China also advanced.
Osaka struggled with unforced errors and had four of her five double faults in the first set. She won the first game with the help of three unforced errors from Kanepi, but the four-time Grand Slam champion lost her momentum after going up 2-1.
Kanepi won four of the next seven games to go up 5-3. From there, Osaka had to fight her way out of two set-point opportunities from Kanepi.
With a mix of a strong backhand and multiple errors from Kanepi, Osaka was able to tie it up and send the set to a tiebreaker.
With the tiebreaker tied 3-3, Kanepi turned up her level of play taking four of the next five points as Osaka handed her set point on a shot past the baseline.
Kanepi built on that momentum in the second set, taking advantage of Osaka's mishaps and firing sharp forehands past the Japanese star.
After hitting her second ace of the match earlier in the final game, Kanepi closed out with a strong serve that Osaka returned high and wide.
Earlier in the day, Canada's Rebecca Marino fell 6-3, 6-7 (5), 4-6 to Zheng Qinwen.
Marino, who made it into the WTA top 100 rankings for the first time since 2012 and is currently No. 96, got rolling early as she took the first set with relative ease.
The Vancouver native overcame a strong start from Zheng in the second set, but had trouble with unforced errors in the tiebreaker.
Marino went up 4-3 in the final set before losing the final three games.
Fellow Canadian Carol Zhao also dropped her first match, 6-1, 6-3 to American Amanda Anisimova.