Defending champion Venus Williams was a shock first-round casualty at the WTA ASB Classic, ousted 6-7 (4-7) 6-3 6-3 by Russian teenager Daria Kasatkina in Auckland on Tuesday.

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 Kasatkina - ranked 75th in the world.

Williams won a first-set tie-break, however, 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."

However, 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.