Overcoming a right wrist issue, Novak Djokovic secured victory against the Czech Republic's Jiri Lehecka with a scoreline of 6-1, 6-7 (3), 6-1, enhancing Serbia's chances to advance to the quarterfinals of the United Cup mixed-team event.
Djokovic, holding the top spot in the rankings and being the defending champion of the Australian Open, faced trouble with his wrist during Tuesday morning's practice.
The injury necessitated intensive treatment from his physiotherapist.
Djokovic was cleared to play his singles match, and it was just as well after Wimbledon champion Marketa Vondrousova defeated Olga Danilovic 6-1, 3-6, 6-3 to give the Czechs a 1-0 lead.
The 24-time Grand Slam winner was on track for an easy victory against Lehecka after winning the first set and leading 3-1 in the second. But with his wrist troubles growing as the match wore on, Djokovic started making a series of uncharacteristic unforced errors.
Djokovic took a medical timeout after losing the second-set tiebreaker, but he came out strongly with a double break in the third set to seal victory.
"I managed to play through," Djokovic said of his injury. "It's not the first or last time; these things happen. You have to find a solution, and thankfully, I managed to finish the match. Let's see what happens now."
Officials first said Serbia's quarterfinal spot was guaranteed, but confusion reigned after the match as to whether Serbia had already done enough to qualify.
The initial declaration was that even if Serbia went on to lose the mixed doubles and, therefore, the tie, it would qualify for the quarterfinals as the best second-place team in Perth.
But organizers later clarified if Serbia loses the mixed doubles in straight sets, the team is out. If Serbia loses in three sets, then it will come down to a games-won percentage.
In the first of two ties in Sydney on Tuesday, Chile defeated Greece in the mixed doubles to claim a 2-1 win.
Stefanos Tsitsipas – the runner-up at last year's Australian Open – was scheduled to face Chile's Nicolas Jarry in the men's singles match but pulled out at the last minute with a back injury. He was replaced by 19-year-old Stefanos Sakellaridis, who lost to Jarry 6-3, 3-6, 7-5 to level the tie.
But Chile won the mixed doubles in three sets to move into first place in its group ahead of Canada, which plays its final group game on Wednesday against Greece. Tsitsipas and Sakkarri lost 6-7 (5), 6-3, 10-6 to Daniela Seguel and Tomas Barrios Vera.
Greece needs to beat Canada to have any hope of advancing to Thursday's quarterfinals in Sydney.
Maria Sakkari had given Greece a 1-0 lead with a 6-0, 6-1 victory over Seguel. Sakkari took just 26 minutes to wrap up the first set in the morning session at Sydney's Ken Rosewall Arena.
"I don't feel match-sharp, but I feel good," Sakkari said. "I'm very happy to have given my team the first point and I have full belief in the team, so I'm super excited for what's coming."
The Netherlands, 1-0, was scheduled to play Croatia, 0-1, in the late match in Sydney.