Stefanos Tsitsipas is returning from an elbow injuryWorld number four Stefanos Tsitsipas admits he is unsure if he will be fit to play at the Australian Open.
Greece's Tsitsipas was beaten 6-7 (5-7) 6-3 6-3 by Argentina's Diego Schwartzman at the ATP Cup in his first singles match since elbow surgery.
He had sat out his first singles match at the tournament in Sydney to "protect" his elbow.
But after playing in the doubles on Saturday, he conceded he was "in a lot of pain".
The Australian Open starts on 17 January in Melbourne.
"That is enough [time to recover], I believe," he said. "But I haven't had [this kind of injury] before, so I don't know. It's my first time experiencing something like this."
Tsitsipas won the first set against Schwartzman, before the 29-year-old rallied to win the match and secure victory for Argentina.
Earlier, his team-mate Federico Delbonis beat Greece's Michail Pervolarakis 7-6 (7-5) 6-1.
"I was just trying to think that it was his first match for two months after his elbow [injury], so I was thinking just to try and be in the match," said Schwartzman.
"I had the chance and I think I was ready. I am very happy because playing against Stefanos and being a set and a break down is not easy, but I found a way."
Argentina will face Poland on Wednesday in their final group D match with a place in the semi-finals up for grabs, after Poland beat Georgia 3-0.
World number nine Hubert Hurkacz, from Poland, recovered from a set down to beat Georgia's Aleksandre Metreveli, the world number 570, 6-7 (5-7) 6-3 6-1.
Pablo Carreno Busta and Roberto Bautista Agut won their respective singles ties in straight sets to ensure victory for Spain over Norway.
Bautista Agut upset Norwegian world number eight Casper Ruud 6-4 7-6 (7-4) after Carreno Busta had beaten Viktor Durasovic 6-3 6-3.
Chile's match with Serbia will go to the deciding doubles with the score tied at 1-1 after the singles rubbers.
After Filip Krajinovic defeated Alejandro Tabilo 6-4 3-6 7-6 (7-5) to give Serbia the early advantage, team-mate Dusan Lajovic was forced to retire injured from his match with Cristian Garin with the Chilean winning 4-6 6-4 3-0.
