Venus Williams, Ana Ivanovic crash out in Auckland

Much had been expected of top seed and seven-time grand slam champion Williams, who boasted a 9-1 win-loss record at the tournament after taking out last year's final.

But Williams and her bid for back-to-back titles came unstuck in an error-riddled performance against unheralded 18-year-old Daria Kasatkina.

The American was beaten 6-7 (4-7) 6-3 6-3 by the Russian teenager, who is ranked 75th in the world.

Williams won a first-set tie-break, but it was all downhill from that point as Kasatkina regrouped to triumph in two hours and 17 minutes at the International series event.

"She is an amazing tennis player and I respect her so much," said Kasatkina, who will meet Nao Hibino for a place in the quarter-finals. "It was an unbelievable match for me."

The shocks continued later in the day, as British qualifier Naomi Broady ousted world number 16 Ivanovic in straight sets.

Broady, ranked 106 places below the former French Open winner, broke the Serbian four times throughout and did not give up her crashing serve once in the second set, as she sent down to 14 aces in total to dispatch the 2014 champion 7-5 6-4 in one hour and 23 minutes.

Bucking the trend of big-names upsets, third seed and former world number one Caroline Wozniacki managed to win through to the second round after seeing off Danka Kovinic 6-4 6-4.

Wozniacki, who lost to Williams in last year's decider, was forced to dig deep against the combative Kovinic, having trailed 4-0 before winning six consecutive games to claim the first set.

The two-time US Open runner-up and Kovinic exchanged breaks late in the second stanza before the former eventually closed out the match.

Next up for Wozniacki is Christina McHale, who defeated Misaki Doi 6-2 6-4.

Fellow seed Barbora Strycova advanced courtesy of a 6-2 6-4 win over Kiki Bertens, former French Open champion Francesco Schiavone bowed out of the tournament, beaten 7-6 (7-3) 4-6 6-3 by Austrian qualifier Tamira Paszek, qualifier Kirsten Flipkens edged Bethanie Mattek-Sands 3-6 6-4 6-3, while Lucie Hradecka fell 6-0 6-3 to Julia Goerges.

In the final match of the day, home favourite Marina Erakovic overcame Yulia Putintseva 7-6 (7-2) 7-6 (7-3).