Hyeon Chung was stunned by world number 310 Rubin Statham at the Auckland Open, while the Sydney International's sixth seed Lucas Pouille suffered a first-round exit.
South Korean Chung, who reached the Australian Open semi-finals last year, fell at the first hurdle for the second tournament in succession, going down 7-5 6-3 to a player ranked 285 places below him.
Despite falling 5-1 behind in the first set in New Zealand, Statham reeled off six games in succession to move ahead and a single break in the second proved enough to delight his home crowd.
The wildcard, who will face Jan-Lennard Struff in the second round, said: "There is a little bit more pressure but there is also an energy behind you and a good following, so when you start playing well, they really get behind you and to be able to play in front of fans and family who have been supporting me throughout the years, it's been really special."
Four-time champion David Ferrer broke Robin Haase six times en route to a 6-2 6-1 victory, while Leonardo Mayer ousted ninth seed Steve Johnson 7-5 6-3.
Peter Gojowczyk beat Guido Pella 6-1 3-6 6-2 to set up a meeting with second seed Fabio Fognini in round two and Pablo Cuevas went down to 7-5 6-1 to Ugo Humbert.
In Sydney, Pouille's first match of the season ended in a 6-2 6-3 loss after just 66 minutes to Andrey Rublev.
Russian qualifier Rublev dropped just two points behind his first serve as he booked a date with Yoshihito Nishioka in the second round.
John Millman fought from a set down to defeat Frances Tiafoe 4-6 7-6 (7-5) 7-5 and his fellow Australian Jordan Thompson also required three sets to advance past Taro Daniel and into the quarter-finals.
Eighth seed Andreas Seppi overcame Jeremy Chardy 7-5 2-6 6-4, while there were wins for Guillermo Garcia-Lopez and Marton Fucsovics.