Naomi Osaka

PERTH, Australia — Four-time Grand Slam singles champion Naomi Osaka is feeling under the weather at the United Cup.

After losing her first match for Japan against Greece’s Maria Sakkari on Friday, Osaka said she got “really sick” over the Christmas holiday period and wasn’t operating at 100 per cent during the 6-4, 6-2 loss.

Osaka was coughing at times during the match and appeared to lack energy. She said she likely caught an illness from her two-year-old daughter Shai.

“I have been dealing with some health stuff...