Serena Williams' Australian Open preparation suffered a jolt with fellow American Madison Brengle stunning the world number two 6-4 6-7(3) 6-4 in the second round of the Auckland Classic on Wednesday.
Playing her first tournament since her semi-final loss at the U.S. Open last September, Serena was left to rue the 88 unforced errors she made in the contest against an opponent 70 rungs lower in the WTA rankings.
"I really think I played - I'm trying to think of a word that's not obscene - but that's how I played," she said after the loss less than two weeks before the year's first grand slam at Melbourne Park.
"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," said the 35-year-old who had constant trouble with her ball toss in the swirling wind.
Brengle was trailing 1-4 before she staged a brilliant comeback to run away with the first set against the 22-times grand slam singles champion who had triumphed 6-0 6-1 in their only previous meeting in 2015.
Serena was a set and 4-3 down when she made a comeback of her own to clinch the tie-breaker and force the decider.
The third set went on serve until the 10th game when Brengle held two match points, only for Serena to fight off both and slump in relief when she hit a crosscourt forehand to get back to deuce.
Brengle strode confidently across court, secured her third match point and stared in disbelief as Serena double-faulted to hand her the biggest victory of her career.
"I've never returned like that in my life. It's a little frustrating, especially since I worked so hard in the off-season, but at least the conditions won't be like this in Melbourne," Serena said.
"She obviously did a much better job than I did. I really abhorred the conditions. I don't think I've ever hit 88 errors in my career. I could google that and that's probably accurate. So there's no need to look back on such a crappy game."
Brengle meets seventh seed Jelena Ostapenko in the quarter-finals.
"Honestly I don't know (how I won)," the 26-year-old American said. "It's so cool. It's what you dream about, playing the best in the world."
Earlier, Serena's sister Venus downed local favourite Jade Lewis 7-6(2) 6-2 before withdrawing with a right arm injury.
"I love this tournament and I'm really sad that I have to withdraw, but I'm happy to have had the opportunity to be here again this year," Venus said in a statement ahead of her match against Japanese youngster Naomi Osaka.
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