ONTREAL, July 29, 2025 (BSS/AFP) - Japan's Naomi Osaka, a four-time Grand Slam singles champion, will work with Polish coach Tomasz Wiktorowski on a trial basis after splitting with Patrick Mouratoglou after less than a year together.

Osaka announced the parting with Frenchman Mouratoglou, the former coach of superstar Serena Williams, on social media on Monday before defeating Canadian qualifier Ariana Arseneault 6-4, 6-2 in 76 minutes in a first round match at the WTA Canadian Open in Montreal.

Osaka, ranked 49th, began working in Canada with Wiktorowski, who coached Poland's Agnieszka Radwanska from 2011-2018, helping her reach the Wimbledon final and world number two ranking in July 2012.

Wiktorowski also coached Poland's Iga Swiatek from late 2021 until last October, a run where she won four of her six career Grand Slam singles titles, before adding the most recent at Wimbledon earlier this month.

Osaka, who improved to 11-1 in matches against players ranked outside the top 100, will next face 13th seed Liudmila Samsonova in the second round. They have split four prior meetings but Osaka won the two they played on hardcourts.

Former world number one Osaka, 27, began working with Mouratoglou last September.

"Merci Patrick it was such a great experience learning from you," Osaka posted on social media. "Wishing you nothing but the best. You are one of the coolest people l've ever met and I'm sure I'll see you around."

Mouratoglou said on Instagram he was "grateful for the trust, the journey and what we have built together," and told Osaka he "will always root for you."

Under the Frenchman's guidance, Osaka reached the Auckland final in January but retired after the first set with an abdominal injury. She won twice at the Australian Open but retired after the first set in her third match with the same injury.

Osaka won her first WTA title since the 2021 Australian Open in May at the L'Open 35 de Saint Malo, but fell in the first round of the French Open and the third round at Wimbledon.