Serena and Venus Williams each saw their run at the ASB Classic come to an end on Wednesday.
Venus had to withdraw due to an arm injury after playing the first of her two scheduled matches for the day, but Serena was stunned in windy conditions by fellow American Madison Brengle.
Serena had raced into a 4-1 lead in the first set, but clunky footwork in blustery weather cost her as Brengle won five successive games to seize the initiative.
The secret to Brengle's success? Playing terribly, she claimed.
She asked coach Nicole Melichar at one changeover: "I feel like if I play even uglier off the return is that a good idea?
"I feel like if I'm actually hitting it she's like 'oh yeah this is nice' and she hits a winner.
"So maybe like even worse and she'll be surprised at how bad I am."
Brengle saved five set points in the second set, which Serena won on a tie-break, but she maintained enough fight to see off the former world number one in the decider and book a meeting with Jelena Ostapenko, a straight-sets winner against Mirjana Lucic-Baroni.
Serena's sister Venus had her schedule affected by rain in New Zealand, and she could not live with two games in a day, pulling out of the tournament after a win over Jade Lewis had set up a second-round match with Naomi Osaka.
Osaka will be well-rested for her quarter-final against a double winner on Wednesday. Croatia's Ana Konjuh beat Kirsten Flipkens and Yanina Wickmayer.
Caroline Wozniacki is the biggest name left in the tournament, having booked her spot in the last eight by ousting Varvara Lepchenko 6-3 6-3. Julia Goerges awaits for the Dane, having beaten Naomi Broady, who ran out of steam in her second match of the day.
Barbora Strycova and Lauren Davis booked a quarter-final against each other with respective wins over Lucie Safarova and Kurumi Nara.