Auger-Aliassime enjoyed a strong second half of the 2022 season, picking up titles in Florence, Antwerp and Basel, as well as helping Canada to the Davis Cup crown, but failed to reach those levels in a 6-4 7-6(5) loss to Popyrin.
The Australian fired 11 aces against his second-seeded opponent to wrap up the biggest victory of his career. His previous two wins over top 10 opponents, Dominic Thiem and Stefanos Tsitsipas came via retirements.
"I played really well, really solid. I served well and used my forehand when I had to, solid on my backhand ... it was down to all the hard work I did in the off-season with my team," Popyrin said. "I'm just really grateful for this win.
"It's up there (with the other big wins). After a tough season, last year to come out first match and beat a guy like Felix is a great feeling and the hard work is paying off."
Up next for the world number 120 is American Marcos Giron (29), who won his opening match against French veteran Richard Gasquet on Sunday.
Denmark's Rune, who won three titles in 2022 including the Paris Masters, where he beat Novak Djokovic (35) in the final, lost 2-6 6-4 6-4 to Nishioka.
The Japanese player meets American Mackenzie McDonald (27)Mackenzie McDonald next.
Miomir Kecmanovic (23) overcame Australian Christopher O'Connell (28) 6-4 6-4 and could play Daniil Medvedev (26) in the second round if the third-seeded Russian gets past Italian Lorenzo Sonego on Tuesday.
WARM RECEPTION FOR DJOKOVIC
Meanwhile, Djokovic was warmly received by Australian fans as he began his 2023 season partnering Canada's Vasek Pospisil (32) in a doubles defeat by Tomislav Brkic and Gonzalo Escobar.
The Serb was deported on the eve of the 2022 Australian Open for not being vaccinated against COVID-19 and received a three-year travel ban for the country, which was eventually waived in November, allowing him entry.
Djokovic was constantly cheered on by the crowd and remained on court after his match to sign autographs and take pictures with fans.
Djokovic faces Frenchman Constant Lestienne in singles on Tuesday.
