Krajinovic had won the first set 6-3 against the young Italian Sinner, with Djokovic looking unimpeded when he replaced his fellow countryman in front of 4000 fans.
The world No. 1 won the second set 6-3 over Sinner, playing some typically stunning shots in the process.
“I am sorry I didn’t step on the court from the beginning. I had to do some treatment with my physio, I wasn’t feeling my best the past couple of days, I wasn’t sure how I was going to react [to coming out of quarantine],” Djokovic said after the match.
“Having an ATP Cup and Australian Open around the corner, you don’t want to risk it too much. But the emotion was so strong in me to come out on the court today, seeing these full stands I had to play.”
Hours later, there were no mid-match player swaps when Barty played Halep in her first match since the Qatar Open last February.
Barty started in style and appeared set to cruise to a straight sets victory but the match was turned on its head in the second set.
The Australian composed herself and pushed Halep the distance in the tie-break but a forehand error at 8-all provided a match point on Halep’s serve and the Romanian capitalised.
Before Barty blew out the cobwebs, Rafael Nadal and Dominic Thiem showed a few early signs of rust – framing a handful of balls they would have otherwise dispatched for winners – before finding their feet and putting on a show for the Adelaide crowd.
The Spanish star broke Thiem twice and held firm on his own serve on his way to a 7-5, 6-4 win.
Both players were in excellent touch considering they have only just emerged from 14-day quarantine.
Earlier, Serena Williams beat Naomi Osaka.
Both women shook out their cobwebs in a light-hearted affair, with Williams winning in a match tie-breaker 6-2, 2-6 (10-7).
The players in action on Friday, except for Barty, had been some of the lucky few to complete their mandatory hotel quarantine in Adelaide, with generally bigger accommodation and more people in their touring entourages. They were all released on Friday.
Williams, who spoke earlier in the day, said she took her young daughter to the zoo after being set free, while Djokovic could be seen taking a barefoot walk through the park.
“These guys and girls will definitely have a disadvantage,” Djokovic said of the 72 players who endured hard lockdown in Melbourne because of a positive COVID-19 test on their charter flight.
“They’ve done their best to do some training in their rooms, but there’s not much movement — they couldn’t hit the ball,” he said.
Williams said she was happy to be out in the open, given time inside quarantine with a child was demanding.
“Yeah, we went to the zoo because we had a calendar in our room and every day we marked an ‘X’ on the days that went by and a big circle on quarantine ending day and we promised we would take her to the zoo to see koalas and kangaroos,” she said.
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Anthony is a sports reporter at The Age.
Sam is a sports reporter for The Sydney Morning Herald.