Ash Barty crashed out of the Adelaide International at her first hurdle after going down in straight sets to Danielle Collins.
It took only 65 minutes for American Collins to defeat the defending champion and world number one in a 6-3 6-4 triumph.
Barty, who had a bye in the first round, did have opportunities, breaking in the first game of the match and building a 4-1 lead in the second set.
But on both occasions Collins stormed back, claiming her first win in four attempts against home favourite Barty, who had beaten her in the semi-finals of last year's tournament.
Coco Gauff continued her fine run to reach a first WTA quarter-final since August 2020 with an impressive comeback win over sixth seed Petra Martic.
The American triumphed 5-7 6-3 6-4 in two hours and 12 minutes to book a clash with compatriot Shelby Rogers at the WTA 500 event.
Gauff did not relinquish serve once after losing the first set and did not even offer up a break-point chance in the decider, which she sealed with a pivotal early break secured with a passing winner.
"I go into every tournament thinking that I can win," said the 16-year-old.
"Obviously that's impossible, to have a perfect record, but I go into every tournament believing that I can win.
"I try not to think far ahead in the match, at least I just try to focus on my next match, just because you have to get through that before you can get to the finals, and just take it one match at a time."
Second seed Belinda Bencic will face Storm Sanders in the quarter-finals after a 6-1 6-3 win over Misaki Doi.
In a match where she was not quite as dominant as the scoreline suggests, Bencic had to fend off 12 break points but was successful in saving 10 of them.
Number four seed Elise Mertens withdrew before her match with a shoulder injury, so Anastasija Sevastova beat lucky loser Christina McHale to set up a last-eight tie with Jil Teichmann.
ATP Finals: Sinner happy to come through 'very tough match' against Fritz
Jannik Sinner insisted he was prepared for the "aggressive" Taylor Fritz in their first meeting since the US Open final in September at the ATP Finals on Tuesday.
Sinner improved his record over Fritz to 3-1 with a 6-4 6-4 victory in Turin, going level with Alexander Zverev with a Tour-leading 67th victory of the season.
The Italian now sits at the top of the Ilie Nastase Group after two games, and faces Daniil Medvedev on Thursday, with the Russian beating Alex de Minaur in straight sets.
"It was a very tough match. We got to know each other from a grand slam final," Sinner said.
"We knew exactly what to expect today. He was very aggressive, but I was prepared too. I just tried to serve well in the important moments.
"In the return positions, his quality of tennis was very, very high, so I was struggling to return his serve, but I was happy with the situation.
"It was a very important day for me today, and I'm very happy that I got through."
Sinner fought off tough resistance from Fritz in front of a home crowd in Turin, hitting 22 winners compared to his opponent's 19.
Fritz had initially earned the first break point of the match, but was unable to make it count before fighting back from 0-40 to level the first set at 4-4.
Sinner ultimately piled the pressure on to win the first set and capitalised on a double fault in the second to break Fritz and win. Fritz will now face de Minaur on Thursday.
ATP Finals: Sinner stays perfect with assured Fritz victory
Jannik Sinner continued his quest to end his stellar season with the ATP Finals title after downing Taylor Fritz in straight-sets to earn his second win of the tournament.
Sinner emerged a 6-4 6-4 victor in an hour and 40 minutes in Turin, with his latest triumph improving his record to 7-0 on indoor hard courts in 2024.
The Italian showed his intentions with a love service game to open the first set, but was matched by his American opponent, who was also looking to maintain his perfect start.
Sinner missed four break points in the eighth game, but made no mistake in Fritz's following service game to claim the early advantage in Turin.
Fritz again traded blows with the year-end world number one, but would again lose a decisive break late in the second set that put Sinner within touching distance.
The American exchanged in another pulsating rally with his opponent, but sent a backhand wide to send Sinner through and to the top of the Ilie Nastase Group after two matches.
Up next for Sinner is Daniil Medvedev, who beat Alex de Minaur earlier on Tuesday, while Fritz will face the Australian on Thursday.
Data Debrief: The Italian Job
In a back-and-forth tussle with Alexander Zverev to end the year with the most wins, Sinner levelled the German with his Tour-leading 67th victory in 2024 on Tuesday.
He also sealed a 12th win against an American opponent this season, only managing more wins against Ben Shelton (three) this season than Fritz (two, level with Alex Michelsen).
Sinner (2023-24) is only the third player in the Open Era to claim 47+ ATP match wins on hard court in consecutive seasons after Roger Federer (2005-06) and Novak Djokovic (2012-13, 15-16).
Daniil Medvedev credited his ability to "block out the noise" as crucial in getting his ATP Finals campaign up and running with a straight-sets victory over Alex de Minaur.
Medvedev, who lost his opening game of the tournament to Taylor Fritz, returned to winning ways with a 6-2 6-4 triumph over the Australian in 78 minutes on Tuesday.
The Russian cut a frustrated figure in his defeat to Fritz on Sunday, breaking his racket and was docked a point after serving a third consecutive double fault during the encounter.
However, the fourth seed produced an expertly measured performance, winning 20 out of 31 baseline points in the opening set to claim the early advantage.
The second set was more evenly contested, but Medvedev secured a crucial break point in the ninth game, going on to seal his first top 10 win on hard courts since March.
"After the last match I was too tired to fight mentally the way I do and so, during the match, when it doesn't work your way, you want to win and when you start losing you get frustrated," Medvedev said.
"So I went into this match thinking, if I lose I go home on Thursday for sure and if I win then I have the chance to have a good feeling. I tried to hit some shots and it worked well and I'm ok with it.
"A lot of football players do it [block the noise]. It's more about social media and for sure after my performance with Taylor and some of my comments ... I tried to read it less.
"I went into this match trying to block the noise even from myself, so no tantrums and I really didn't care what was happening on the court."
Medvedev will seek to bolster his hopes of progressing from the John Newcombe Group when he takes on Jannik Sinner next, with the Italian in action against Fritz later on Tuesday.
Data Debrief: Cool, calm and collected
Medvedev kept his hopes of sealing a second ATP Finals crown alive with a composed display against De Minaur, improving his record to 45-19 for the season.
While the Russian produced more unforced errors (18) than De Minaur (15), Medvedev showed his quality by hitting 24 winners, 14 of which came with his forehand.
Medvedev also claimed his sixth top 10 win of 2024, while also improving his head-to-head record against his Australian opponent to 7-3, avenging his defeat in the French Open round of 16 earlier this year.
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