World No.16 Osaka was coughing at times during her 6-4 6-2 loss at RAC Arena in Perth on Friday night.
Stefanos Tsitsipas, now ranked 36th in the world following 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 world No.99 Shintaro Mochizuki.
Osaka seemed to lack energy at times in Friday night's match, and she later revealed that an illness she likely caught from her daughter around Christmas Eve was still hanging around.
"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 per cent 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, so hopefully it goes away before the Australian Open."
Osaka struggled with her serve early, dishing out three double faults as well as three 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.
The 28-year-old immediately broke back, and the rest of the first set became a slog before Sakkari set up two set points while leading 5-4.
Osaka, who made the US Open semi-finals last year, produced two strong serves to save her skin, but Sakkari gave herself a third set point with a strong backhand down the line.
On this occasion Osaka hit her overhead shot long, handing Sakkari the opening set in 57 minutes.
The second set lasted only 37 minutes, with Sakkari racing to a 5-1 lead before overcoming a break of her own serve to seal victory.
Despite the disappointing loss, Osaka still stayed on court for a while to sign autographs and take selfies with fans.
World No.52 Sakkari was thrilled with the win, especially after a year in which she plummeted down the rankings.
"Naomi is one of the best players in the world, regardless of her ranking," Sakkari said.
"She had a great end of the season last year. She's one of the most dangerous players and a player that everyone respects.
"So you always have to try to play the best you can, because she's very powerful.
"She has a lot of weapons, so I was very happy to play the way I played."
In Friday's earlier tie, Argentina beat Spain 3-0 after Sebastian Baez and Solana Sierra romped to singles wins.
World No. 66 Sierra beat Jessica Bouzas Maneiro 6-4 5-7 6-0 after Baez had defeated Jaume Munar 6-4 6-4.
