World number two Williams was beaten 6-4 6-7 (7/3) 6-4 by the 26-year-old, who is 70 places below her in the current rankings
Top seed Serena Williams was knocked out of the ASB Classic in Auckland in the second round by fellow American Madison Brengle on Wednesday.
World number two Williams was beaten 6-4 6-7 (7/3) 6-4 by the 26-year-old, who is 70 places below her in the current rankings.
It was the biggest win of Brengle’s career and her first against a top-three opponent, with Williams making 88 unforced errors.
“I really think I played – I’m trying to think of a word that’s not obscene – but that’s how I played,” Serena told wtatennis.com after the match.
“Eighty eight unforced errors is too much, just way too many. It’s a lot. I can’t expect to win hitting that many errors.
“I couldn’t get used to the wind; my opponent was playing in the same conditions, and maybe the wind was more suited for her game, but it was really annoying me for whatever reason.”
Brengle will now face seventh seed Jelena Ostapenko, who dispatched Mirjana Lucic-Baroni 6-2 7-3 (7/3).
There was no such problem for Serena’s sister Venus, seeded second, as she saw off wildcard Jade Lewis 7-6 (7/2) 6-2 in her opening match.
Caroline Wozniacki eased into the quarter-finals after a comfortable straight-sets win over Varvara Lepchenko.
The third seed won 6-3 6-3 in an hour and 17 minutes to line up a last-eight match with German Julia Goerges, who beat Britain’s Naomi Brody 7-5 6-4.
Both Goerges and Brody had played matches earlier in the day as poor weather earlier in the week forced them to double up. Goerges defeated sixth seed Anastasia Pavlyuchenkova in the first round while Brody beat Danka Kovinic.
Barbora Strycova, Lauren Davis and Ana Konjuh all came through their second-round matches. Konjuh had earlier seen off eighth seed Kirsten Flipkens in the first round.
In the Shenzhen Open, second seed Simona Halep was beaten 6-3 4-6 7-5 by Katerina Siniakova while fellow Czech Kristyna Pliskova beat qualifier Kai-Chen Chang 6-1 6-2.
Italian Camila Giorgi beat Zheng Saisai 6-7 (7/5) 7-5 6-4 and Nina Stojanovic won her second-round match against Ons Jabeur.
In the Brisbane International, Roberta Vinci eased through to the quarter-finals with a 6-4 6-2 win over Misaki Doi. Vinci will face Karolina Pliskova in the last eight.
Second seed Dominika Cibulkova also progressed with a 2-6 6-4 6-4 comeback win over China’s Zhang Shuai while fifth seed Svetlana Kuznetsova beat Destanee Aiava 6-4 6-3.