Tomic clinched his second career win over Djokovic as Australia opened up a lead over top-ranked Serbia in the mixed-teams event, before Ana Ivanovic levelled it up with a 6-2 6-3 win over Ashleigh Barty.
Djokovic then teamed up with Ivanovic to beat Tomic and Barty 10-6 in a champions tie-break to clinch a 6-4 6-7 (8-10) 10-6 win.
The story of the day though remained Tomic's win over Djokovic.
20-year-old from Queensland came into the match having lost all three of his ATP encounters with Djokovic, including their quarter-final match at Wimbledon in 2011.
Tomic, who has slipped to number 52 in the world rankings after reaching a career-high of 27 last June, edged out Djokovic in the opening set before reeling off three winners to break in the second.
It was a huge victory for Tomic, who had only previously beaten the Serb at the Kooyong Classic in 2010, ahead of the Australian Open in Melbourne.
"I'm thankful to get the opportunity to play against Novak," Tomic said. "I was very nervous before the match. I've worked hard on my serve and technique in the off-season and I think that's why I won. And I've been working on my head a lot, trying to not get distracted."