Former world number one Naomi Osaka says she is not at full health following her straight-sets loss to Maria Sakkari at the United Cup.

Just over two weeks out from the Australian Open, Osaka was coughing at times during her 6-4, 6-2 defeat to world number 52 Sakkari in Perth on Friday night.

Stefanos Tsitsipas, now ranked 36th in the world after a 2025 campaign ruined by a back injury, wrapped up victory in the tie for Greece over Japan with a 6-3, 6-4 win against Shintaro Mochizuki.

Osaka lacked energy at times against Sakkari, who has been ranked as high as number three in the world.

Four-time major winner Osaka revealed she thinks she contracted a viral illness from her daughter over Christmas.

"I have been dealing with some health stuff, so I'm kind of just happy to be out here right now," Osaka said.

"It's not serious, but I'm not operating at the [performance level] that I want to be operating at, which kind of sucks because I had a really good off-season, so I thought I was going to do really well here.

"But then I got really sick, so I think I'm at the tail end of that, but still not amazing.

"I'm just trying to get better every day.

"I had a cough, a runny nose, like all that nasty stuff. Hopefully it goes away before the Australian Open."

Osaka struggled with her serve early, dishing out three double faults and a series of failed ball tosses on the way to losing her opening service game.

"I definitely wouldn't blame that on being sick, I was just really nervous, so my hand was shaking a little bit," Osaka said.

Despite the disappointing result, Osaka stayed on court after the match to sign autographs and take selfies with spectators.

In Friday's earlier tie, Argentina beat Spain 3-0 on the back of singles wins to Sebastian Baez and Solana Sierra.

World number 66 Sierra beat Jéssica Bouzas Maneiro 6-4, 5-7, 6-0 after Baez had defeated Jaume Munar 6-4, 6-4.

AAP