World number six Felix Auger-Aliassime began his season with a straight-sets defeat by Australian qualifier Alexei Popyrin on Monday while Holger Rune crashed out to Yoshihito Nishioka in the ATP 250 Adelaide International 1 first round.
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 23-year-old 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.
"Yeah, really good win for me, especially coming off last season where I only had five wins on the circuit. For me it's a very important win," Popyrin told reporters.
"I know I have the level to play against guys like that, and today just proved it, so yeah, I'm happy. I'm not surprised, to be honest, but I'm happy with it."
Up next for the world number 120 is American Marcos Giron, 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 the 19-year-old beat Novak Djokovic in the final, lost 2-6 6-4 6-4 to Nishioka.
The Japanese player meets American Mackenzie McDonald next.
Miomir Kecmanovic overcame Australian Christopher O'Connell 6-4 6-4 and could play Daniil Medvedev in the second round if the third-seeded Russian beats Italian Lorenzo Sonego.
Warm reception for Djokovic
Meanwhile, Djokovic was warmly received by Australian fans as he began his 2023 season partnering Canada's Vasek Pospisil 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.
The 35-year-old 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.
