Coco Vandeweghe rounded off a day of shocks at the Australian Open with an electrifying display of power tennis to overwhelm Angelique Kerber, the top seed and defending champion, 6-2, 6-3.

Kerber, who has endured a miserable start to the year after also losing early at warm-up events in Sydney and Brisbane, had no answer to the 35th-ranked American’s powerful serving and raking groundstrokes, and found herself on the back foot virtually throughout the one hour, eight minute contest. It was, at least, mercifully brief for the German, whose exit in the Rod Laver Arena followed the defeat of fellow world No1 Andy Murray to Mischa Zverev a few hours earlier on the same court.

Vandeweghe has traditionally been seen as a player who blows hot and cold; this was very much a hot Coco. Summoning a level of consistency that has too rarely been seen, the 25-year-old bludgeoned down six aces and 30 winners to match her best previous showing at a major, a run to the last eight at Wimbledon in 2015. Along the way, her ebullient if intense disposition, all frequent smiles and raging zeal, afforded a stark and perhaps revealing contrast with the more sombre Kerber, who appears to be finding life at the top heavy going. Win or lose, the German seems a million miles from greeting any result with a casual shrug, as Vandeweghe did here.

“I wasn’t feeling confident, I guess I faked it. I was nervous, but I had a game plan to execute and I knew that as along as I keep picking my spots, I had a chance,” said Vandeweghe.

“But, you know, fake it ‘til you make it’ … my game-plan was to execute knowing that she was going to get a lot of balls back.”

Coco Vandeweghe said she was feeling ‘crap’ despite winning. Photograph: Mark R. Cristino/EPA

In the event, Kerber struggled to find her trademark consistency and, moving sluggishly, was repeatedly caught at her feet by the New Yorker’s deep, penetrating returns.

“I made a lot of mistakes, said Kerber, who will lose her No1 ranking should second seed Serena Williams, who plays the wily Czech Barbora Strycova in the fourth round, goes on to claim a seventh title. “I think that was not my game I play normally. It was a tough match and, of course, I’m disappointed. But I was not feeling the ball at all tonight. I was not playing good from the first point. It was not my day and not my match.”

For Vandeweghe, who said she had set herself a target of making the last eight before the tournament began, it is a case of onwards and upwards. She will now face Garbiñe Muguruza, the reigning French Open champion and seventh seed. The Spaniard, the highest seed remaining in the upper half of the draw following Kerber’s defeat, beat the 78th-ranked Sorana Cirstea, of Romania, 6-2, 6-3.

“I know grand slams are very important and when you win one you have that belief in yourself that you can do it again,” said Muguruza.

Also looming large in the top section is Venus Williams, who saw off Germany’s Mona Barthel, a qualifier ranked 181st, in straight sets, 6-3, 7-5. The American, at 36 the oldest player left in the draw, has yet to drop a set.

Williams will next play Anastasia Pavlyuchenkova, the 27th-ranked Russian,who beat compatriot Svetlana Kuznetsova, the eighth seed, 6-3, 6-3. Kuznetsova survived a three-and-a-half-hour marathon against Jelena Jankovic in the previous round, but refused to blame her defeat on fatigue. “I cannot say I was very tired, but I was really tight. I mean, definitely I was not the freshest, but still, I was OK,” she said.