Two days after Djokovic sustained a minor injury when a crowd barricade fell into his shin while he was signing autographs, Djokovic was defeated 6-4 6-4 as Australia [ Images ] took a 1-0 lead in the tie against Serbia.
Australia's Ashleigh Barty was due to play Ana Ivanovic [ Images ] in the women's singles later.
Tomic had a poor 2012 in which he fell to outside the top 50 in the world rankings but the 20-year has vowed to return to the top 10 this year.
"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."