Naomi Osaka, this year's U.S. Open women's singles champion, on Monday donated 30 tennis rackets to children in Hokkaido, which was hit by a major earthquake in September.

Osaka's grandfather Tetsuo, 73, who lives in Nemuro in the northernmost prefecture, attended a presentation ceremony at the prefectural government office on her behalf.

"Naomi has been worried about the damage from the earthquake and wants to be of help to Hokkaido. We hope children in the affected areas will enjoy playing tennis," Tetsuo said as he handed one of the rackets to Hokkaido Gov. Harumi Takahashi.

Osaka, who started this year at No. 68 in the WTA rankings, rose to fourth in the world in October, tying the all-time record for the highest-ranked Japanese player, male or female.

She won the U.S. Open in September when Hokkaido was rocked by a magnitude 6.7 earthquake that left more than 40 dead.

The daughter of a Haitian father and Japanese mother, Osaka was raised and resides in the United States. She has dual Japanese and American citizenship.