John Higgins, the world champion, said he "needs to knuckle down" after his shock first-round elimination from the Australian Goldfields Open in Bendigo.
Having taken a 4-1 lead, the Scot lost 5-4 to Matthew Selt, a player who has never previously been beyond the last 32 of a ranking tournament.
Higgins was also one half of the Scottish team who suffered a World Cup quarter-final loss to Northern Ireland, and after Tuesday's setback he hinted at a lack of focus since his Crucible success in May.
"I'm disappointed but maybe it's because I haven't put in the hours practising since winning the world championship," he said. "It doesn't matter if you're world champion or world number 300, you need to put the work in and if you don't, you don't get anywhere.
"I've not started the season yet. It's only been a short time since the end of the last one and I've been spending it with my family but I need to knuckle down."
After losing the first frame, Higgins seemed to take charge by claiming the following four to take a commanding 4-1 lead. However, the 26-year-old Selt showed fighting spirit to turn the match on its head and win four in a row himself despite managing a highest break of only 63.
A delighted Selt, who now faces the seven-times world champion Stephen Hendry in the last 16, said: "John Higgins is the greatest player of all time and Stephen Hendry is the greatest winner of all time.
"I knew I could compete at this level and, career-wise, beating John is a great achievement. I've just won my first match on TV and now I feel like a proper player. If I play well this week I can go all the way and this has given me the belief that I can do it."
Ali Carter was another player to fall victim to an upset, losing 5-3 to Marcus Campbell despite leading 2-0 after a sparkling 134 in frame two. The England World Cup player also had an explanation for his exit and said: "Don't get me wrong, it's great we have all these tournaments, but I can't wait to get home to be honest.
"I've a boy who is 18 months old and I've not seen him for three weeks so that's more important to me, to get home and be with my family."
Neil Robertson, the 2009 world champion, ensured a continued presence in the tournament for the host nation after defeats for his World Cup team-mate Steve Mifsud and his namesake James. Robertson beat Nigel Bond 5-2, making two half-centuries and two more breaks in the 40s.
He said: "This is my first time playing a professional ranking tournament in my country and I'm relieved to get through. It would've been a disaster if all three Australians had gone out in the first two days.
"If I won this tournament it would boost the popularity of Australian snooker and the turnout for the first round was pretty good. Hopefully we can get some massive crowds and create a fantastic atmosphere for my next match."
In the day's other games Mark Allen, beaten along with his Northern Ireland team-mate Gerard Greene by China in Sunday's World Cup final, saw off Wales's Ryan Day 5-3 in a high-quality encounter. The Antrim left-hander edged a tense opening frame and made 60 to go 2-0 up, but Day responded with 96 and 53 to level at the mid-session interval.
Allen produced a run of 89 to win the first frame back but Day levelled in scrappy fashion and took a 47-point lead in frame seven. Allen battled back to win that one, though, and took the match with a break of 72.
Dominic Dale beat Mifsud 5-2, also in the last 32, while Rory McLeod was brought crashing down to earth after Monday's victory over Peter Ebdon as he was thrashed 5-1 by Shaun Murphy in the first last-16 match.