Top seed Agnieszka Radwanska's title defence at the Sydney International lasted just 92 minutes when she was dumped out of the second round by Bethanie Mattek-Sands on Tuesday.
The world number five, who had won the Sydney title last year as part of a 13-match unbeaten streak that ended in the Australian Open quarter-finals, was upset 7-5 6-2 by the American qualifier.
"She was really playing aggressive," Radwanska who was suffering from a slight shoulder injury, told reporters.
"Didn't matter if it was my first serve, second serve, or anything, she was really going 100% for every ball."
Radwanska had a poor end to the season in 2013, losing all three round robin matches at the WTA championships though she won all three matches at the Hopman Cup in Perth last week.
"I really had a lot of good matches last week," the 24-year-old Pole added.
"It's not like I'm playing bad. I'm not worried about that. I just think not my day (and)... sometimes you have to lose.
"I think every week is different story. You start over and over again.
"Every tournament isn't the same thing. You're not winning every week every match. Normally you just to have to start again, not really think what was happening in the last week."
Mattek-Sands will now face fellow American Madison Keys in the quarter-finals.
"She's a great upcoming player, plays aggressive shots, big serve, we've had some battles in the past, so I'm looking forward to tomorrow," Mattek-Sands said of Keys.
Former Wimbledon champion Petra Kvitova will play either Danish sixth seed Caroline Wozniacki or Lucie Safarova in the quarter-finals after the Czech smashed American qualifier Christina McHale 6-1 6-0 in a little over an hour.
"I'm feeling much more better than last year and the results show it," Kvitova said in reference to her first round loss at Sydney last year.
"I'm glad how I played today, that I won my match. That's the most important thing to did here.
"I was really glad I started here better than the last year. I didn't have really nice memories from the last year here.
"So it's important to be in the next round."
Fellow former Wimbledon champion Lleyton Hewitt, however, disappointed Kooyong Classic fans with his decision to withdraw from the invitational tournament in Melbourne.
The 32-year-old caused a major upset at the Brisbane International defeating Roger Federer in the final on Sunday and the Australian said he felt he would need the week off to recover properly for the season-opening grand slam.
"At the moment it's just all about recovery for me," Hewitt told reporters at Kooyong on Tuesday. "When you get older it gets a bit tougher than when you were 16.
"We're going into the first major and you've got to do what's absolutely right for you."
His place in the eight-man field will be taken by Australian teenager Jordan Thompson, though Hewitt will still play an exhibition match against Wimbledon champion Andy Murray at the Kooyong club on Friday instead.
Thompson will play world number nine Richard Gasquet in their first matches on Wednesday. World number eight Tomas Berdych opens the tournament against Spain's Fernando Verdasco.
Verdasco's compatriot David Ferrer was given a traditional Maori welcome at the Auckland Open on Tuesday in recognition of his contribution to the tournament.
Ferrer will be making his 10th appearance at the tournament, having won four titles, including the last three in succession.
"Thank you very much for everything," the world number three told the packed crowd at the Auckland Tennis Centre after being presented with a manaia (Maori carving).
"This tournament is very special. I hope to come back for a lot more years and try to win one more time."
Ferrer will meet qualifier Donald Young in the second round on Wednesday after the American beat Argentina's Federico Delbonis 7-5 6-2.
Former champion Philipp Kohlschrieber also advanced to the second round with a tough 6-2 3-6 7-5 victory over Spain's Pablo Carreno Busta.