Greece make winning start as Baez, Sierra set up a clean sweep for Argentina in opener

Stefanos Tsitsipas found his winning touch on his return to action from injury as he led Greece to a 3-0 win over Japan on Friday in group play at the ATP-WTA United Cup.

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The former world number three dispatched Shintaro Mochizuki 6-3, 6-4 in singles as he played his first match in almost four months after a recurring back injury forced him to take a break from the sport.

"I've had a lot of uncertainty, I didn't know if I could compete," Tsitsipas said. "But to play at this level is something I'm very grateful for.

"I'm healthy and that's the most important thing in the world. I hope this tournament can give me some confidence before the Australian Open (January 18-February 1)," added the 27-year-old two-time Grand Slam finalist.

Teammate Maria Sakkari began the tie with a 6-4, 6-2 defeat of Naomi Osaka.

Japan's former world number one is suffering with a head cold and played far from her best in Perth.

"I was not operating at the percent I would want, I did not play amazing," the four-time Grand Slam champion said.

"Honestly, it was not a bad first match for what it was. I'm thankful I have another to play. I need to not be afraid of making mistakes."