The Bulgarian, who struggled with an ankle injury towards the end of last season, saved six out of seven break points to down his unseeded opponent in the season-opening tournament.
"I felt I was actually hitting the ball quite all right for our first match of the year," former world number three Dimitrov, who has slipped to 19th in the rankings, said.
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"The only thing I can do is play as many matches as I can, and only then I can again establish myself as that top player.
Highlights: Dimitrov defeats Nishioka as 2019 campaign gets underway
"Considering how the past five months have gone for me, there's areas that I need to focus on... to get back to the level that I was."
Up next for the 2017 champion is Australia's John Millman or American Tennys Sandgren.
The big-serving Milos Raonic, another former world number three who slid down the rankings last season to sit one place above Dimitrov, fired 18 aces in his 6-0 6-3 win over Slovenia's Aljaz Bedene.
Dimitrov: Things have been a little rough for me
Canadian Raonic, the 2016 winner in Brisbane, next takes on qualifier Miomir Kecmanovic.
Among the women, 2017 winner Karolina Pliskova rallied from a set and a break down to beat Yulia Putintseva 4-6 6-3 6-4.
"It's always tough to play a first match after one-and-a-half months of preparation and some holidays, so I'm just happy that I'm through," Pliskova said.
"I played good in practice, but still, a match is something different. I'm happy I managed to fight in the second set after being a break down. It was a very important match for me."
Wildcard Kimberly Birrell fought back from 3-5 down in the decider to stun Russia's world number 10 Daria Kasatkina 5-7 6-4 7-6(3) for her first win over a top-10 player.
Fellow Australian Destanee Aiava produced another upset, beating Kristina Mladenovic 6-3 7-6(2) to set up a second round clash with U.S. Open champion Naomi Osaka.
Men's Singles - First Round
 M. Raonic (CAN) d A. Bedene (SLO) 60 63
 G. Dimitrov (BUL) d Y. Nishioka (JPN) 63 64
[Q] M. Kecmanovic (SRB) d L. Mayer (ARG) 63 61
[Q] Y. Uchiyama (JPN) d [Q] U. Humbert (FRA) 64 76(6)
J. Millman (AUS) d T. Sandgren (USA) 76(6) 67(6) 60
Men's Doubles - First Round
 B. McLachlan (JPN) / J. Struff (GER) d [WC] A. de Minaur (AUS) / L. Hewitt (AUS) 63 61
[WC] J. Duckworth (AUS) / J. Thompson (AUS) d K. Nishikori (JPN) / Y. Nishioka (JPN) 76(12) 64
A. Krajicek (USA) / A. Sitak (NZL) d T. Fritz (USA) / N. Kyrgios (AUS) 75 76(4)
M. Daniell (NZL) / W. Koolhof (NED) d R. Jebavy (CZE) / A. Molteni (ARG) 63 57 10-6
Women's Singles - First Round
 K. Pliskova (CZE) d Y. Putintseva (KAZ) 46 63 64
 K. Bertens (NED) d E. Mertens (BEL) 62 67(6) 64
[WC] K. Birrell (AUS) d  D. Kasatkina (RUS) 57 64 76(3)
D. Vekic (CRO) d M. Rybarikova (SVK) 61 62
L. Tsurenko (UKR) d M. Buzarnescu (ROU) 60 62
A. Kontaveit (EST) d C. Suárez Navarro (ESP) 60 63
[Q] H. Dart (GBR) d [WC] P. Hon (AUS) 16 63 64
[Q] D. Aiava (AUS) d K. Mladenovic (FRA) 63 76(2)
Women's Doubles - First Round
 H. Chan (TPE) / L. Chan (TPE) d M. Kato (JPN) / M. Ninomiya (JPN) 61 62
E. Hozumi (JPN) / A. Rosolska (POL) d A. Sevastova (LAT) / E. Svitolina (UKR) 60 64
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