Australian qualifier Alexei Popyrin stunned second seed Felix Auger-Aliassime in the first round of the season-opening Adelaide International on Monday as Novak Djokovic was given a hero's welcome in a rare doubles appearance.
On a day of upsets, world number 11 Holger Rune also crashed out, as did women's third seed Daria Kasatkina.
Canada's Auger-Aliassime was in top form towards the end of 2022, winning three tournaments in October and November before leading Canada to Davis Cup glory when they beat Australia in the final. He rose to number six in the world and said before the tournament he wanted to ride the wave of momentum into the 2023 season. But he came unstuck against Popyrin, who dropped out of the top 100 last year, slumping 6-4, 7-6 (7/5).
The 23-year-old Australian attacked at every opportunity, hitting 32 winners, including 12 aces. Popyrin now plays American Marcos Giron who beat French veteran Richard Gasquet 7-6 (7/5), 6-7 (6/8), 7-5 on Sunday.
Japan's Yoshihito Nishikori downed fifth seed Rune -- the ATP Newcomer of the Year in 2022 after claiming three titles -- in three sets.
Rune, 19, won the first set comfortably but his serve went off the boil and Nishikori came back to win the next two and take the match 2-6, 6-4, 6-4.
Earlier, fans gave Djokovic a raucous welcome during his first match in Australia in almost two years.
Playing with Canada's Vasek Pospisil, they lost to doubles specialists Tomislav Brkic and Gonzalo Escobar 4-6, 6-3, 10/5 but enjoyed overwhelming crowd support.