Australia drew first blood in the Asia-Oceania zone second-round tie against South Korea after Tomic took out Min-Hyeok Cho 7-5 6-3 6-3 at Patrick Rafter Arena.
At just 19, Tomic was asked to lead Australia with Hewitt out until June due to toe surgery.
And a comfortable victory was expected for the world No.36 on Friday.
But the Wimbledon quarter-finalist hardly had everything go his way despite lining up against a player who does not have an official ATP ranking.
Indeed Tomic looked like a concerned man when he was broken in the match's opening game before Cho saved three break points to lead 3-1 in the first set.
Tomic rattled off the next three games before Cho again went toe to toe with the young Aussie from the baseline to provide unexpected resistance.
Cho, 25, only conceded a break in the second game of the second set before giving Tomic another fright in the third.
Tomic broke to lead 2-1 in the third, only to have Cho return the favour immediately then serve out the next game to lead 3-2.
Alarm bells were ringing when Tomic then limped off to receive treatment for what appeared to be a thigh complaint.
However, Tomic regrouped to claim the next four games, sealing his second match point with a backhand winner to extend his Cup singles record to 6-1.
South Korea left out their top six players reportedly due to a dispute with their federation, ensuring the top-ranked player in the no-name outfit that arrived in Brisbane peaked at 730.
World number 18 Australia will advance to the World Group playoffs in September if they defeat No.31 South Korea.
Australia have not been in the elite 16-team World Group since 2007.