SYDNEY, Australia - Angelique Kerber remains unbeaten in 2018 after upsetting No.2 seed Venus Williams 5-7, 6-3, 6-1 in the second round of the Sydney International - a result that means that six of the tournament's seven seeds have lost their opening match.

It was a clash between two former finalists who had never lifted the trophy here. Kerber had fallen short in 2014, when she was runner-up to shock champion Tsvetana Pironkova. Williams had made a splash on her debut as an 18-year-old in 1998, defeating Martina Hingis in the second round and reaching her second ever final (following the 1997 US Open), where she was undone by Arantxa Sánchez-Vicario - but today's match marked her first appearance here of this millennium.

"I was just trying to go for it, to play my game, trying to do what I did good in the last few days," said Kerber. "I was just looking ahead and trying to play point by point."

Both players came out in solid form, but after four uneventful holds it was the American who wobbled in the fifth game, committing her first error at net to fall behind by a break. Kerber continued to find the corners of the court with her forehand for impeccably placed winners, forging a 5-3 lead and then holding two set points on the Williams serve in the following game.

However, the seven-time Grand Slam champion staved off the first with a service winner, then found a spectacular forehand down the line to hold off the second - and from there, began finding lines of her own as her opponent's first serve percentage began a precipitous fall to 48%. 

And it wasn't just Kerber's first delivery causing problems: serving at 5-6, the 2016 Australian Open champion slid to 0-40 courtesy of routine netted groundstrokes, then offered up her fifth double fault of the set to hand the set to Williams.

A cagey start to the second set found both players probing at the other's weaknesses, with Kerber steering the ball from corner to corner and Williams trying to cut rallies short with judicious net forays. Once again, it was the 29-year-old who drew first blood, with a magnificent lob paving the path to a break for 3-2.

With the match peaking, Venus hit back immediately, emerging from a three-deuce battle to square the scoreline after an errant Kerber dropshot - but her own first serve was increasingly awry, landing just 56% of the time in the second set, enabling the German to go after her returns with more intent and break once again.

Up 5-3 in the first, Kerber had let two chances to capture the set go begging. In the same position in the second, she made no mistake. When Williams offered up a set point courtesy of a sixth double fault - a mistake Kerber had by now entirely eradicated from her game - the World No.22 seized it with gusto, hammering a return deep on the baseline to elicit a forehand error.

The deciding set opened with a pair of marathon games that would prove crucial. Williams powered to a 0-40 lead on Kerber's serve in the first game - but a cascade of loose errors meant that she was unable to take advantage. By contrast, despite some spectacular saves by the American on three break points in the subsequent game - a pinpoint drive volley and inside-out forehand winner were particularly remarkable - Kerber finally broke through on her fourth opportunity to move ahead 2-0.

From there, the former World No.1 demonstrated exactly why her resurgent 2018 form has seen her talked up as an Australian Open contender. Her signature forehand down the line, her ability to conjure of wicked angles in the most high-octane rallies and, in the fourth game, a lob of exquisite, pinpoint accuracy were all on display as she stormed to a 5-0 lead.

Competitive to the last, Williams fought off three match points on her own serve to avoid the bagel - but staring down the barrel for the fifth time, sent a backhand return wide to extend the exodus of seeds, and fall to a 3-5 record against the German. Kerber, who ended 2017 on a five-match losing streak, is now on a six-match winning streak - and in the quarterfinals she'll face Dominika Cibulkova, against whom she is tied at 5-5 in the head-to-head, to make it seven.

"It will be another tough battle against Domi," said Kerber. "I mean, we both didn't have the best year last year, but I think we are both looking forward to this year.

"We will just try our best. For me, I can say I'm going out trying to play another good match. Of course trying to win it but just focusing on another day and not looking too much ahead and also not looking back."