Madison Keys' Adelaide International title defence crumbled as she went down 6-4 4-6 6-2 to rising star Victoria Mboko.

Keys triumphed in Adelaide last year before going on to win her first major grand slam title by beating Aryna Sabalenka in the Australian Open final.

The American has not won a title since then, and she will now carry that weight to Melbourne Park after falling to Mboko on Wednesday.

Keys, whose Australian Open campaign will start against Oleksandra Oliynykova, managed to rally in the second set after dropping the opener, but Mboko broke twice early in the decider before closing out victory in under two hours.

"There were ups and downs in the match, I had to weather the storm," said Mboko, who surged up the rankings last year after a breakthrough season in which she won the Canadian Open.

"She won here last year so I know coming in this would be a big fight. I'm glad I was able to get it done.

"Madison is a tough player, she didn't get to where she is without a reason. It's nice to play a former champion to see what I can do."

A breakthrough performance

Victoria Mboko defeats defending champion Keys and books her spot into the final four in Adelaide.#AdelaideInternational pic.twitter.com/QT1RfhdPrd

— wta (@WTA) January 15, 2026

Keys' compatriot Emma Navarro also suffered a quarter-final exit in Adelaide, with the world number 15 losing 6-3 6-3 to Diana Shnaider.

Data Debrief: Errors prove costly for Keys

Keys is 11 years Mboko's senior, yet she could not find her groove and reeled off 41 unforced errors, dropping serve on six occasions.

This marks the second top-10 win of Mboko's career, after the teenager's stunning win over Coco Gauff in Montreal last season.