The British number one saw off his 19-year-old opponent 6-3 6-7 (6) 6-1 in just under two and a half hours to set up a quarter-final against Chile’s Alejandro Tabilo.
Having claimed the opening set, Norrie had match point in the tie-break before Van Assche came through to bring the match level.
He was then broken by Van Assche in the opening game of set three before hitting back emphatically, rattling off six games in succession to secure the win.
Norrie is seeded 19th for the Australian Open ahead of the draw taking place in Melbourne on Thursday.
Fifteen-year-old Gauff - a breakout star in 2019 - is in the same quarter of the draw as 23-time grand slam champion Williams, meaning the pair could meet in the last eight.
Gauff has never faced Serena but stunned her sister Venus Williams at Wimbledon last year.
The teenager took a step towards a first clash with the 38-year-old as she saw off Viktoria Kuzmova 6-3 6-1 in the first round on Monday.
"I felt confident, I feel like I'm moving well," Gauff said, with Laura Siegemund or Alison Van Uytvanck up next. "I had a good off-season, and I think that's showing in my matches."
Williams was not in singles action for the first time this year, however, as opponent Svetlana Kuznetsova withdrew, meaning the American must instead play Camila Giorgi on Tuesday.
The WTA great was still able to thrill the Auckland crowd, as she joined the soon-to-retire Caroline Wozniacki in the doubles, beating Nao Hibino and Makoto Ninomiya 6-2 6-4.
"I had a blast," Williams said. "It's so good to play with her - it's like a dream come true."
Wozniacki, who is calling time on her career at the Australian Open, added: "It's amazing to be able to share the court with Serena."
Elsewhere in the singles, Caroline Garcia recovered from a set down to beat Taylor Townsend, while Eugenie Bouchard, Daria Kasatkina, Tamara Zidansek, Jil Teichmann and Jessica Pegula all also advanced.
There were upsets at the Brisbane International, meanwhile, as Danielle Collins crushed Elina Svitolina 6-1 6-1.
Home hopeful Sam Stosur got the better of fellow former major winner Angelique Kerber, recovering in a tough first set to claim consecutive tie-breaks.
"This is the way I want to be all the time on court," Stosur said. "I put more pressure on myself than anyone could ever do."
Ajla Tomljanovic came through three sets, as did Barbora Strycova against Johanna Konta, with Sofia Kenin and Alison Riske also winning.
Aryna Sabalenka cruised through in Shenzhen, while Garbine Muguruza was made to work for victory against Wang Xinyu.
Wang Qiang and Elena Rybakina ensured all four seeds in action on Monday made it into the second round.
At the first edition of the ATP tournament in Adelaide, sixth seed Cristian Garin fell to Lloyd Harris, the South African qualifier winning 7-6 (7-5) 3-6 6-4.
Jeremy Chardy defeated fellow Frenchman Gilles Simon in straight sets, with Dan Evans, Sam Querrey and home hope James Duckworth the other players to progress.
The action came as top seed De Minaur pulled out of the event after aggravating an abdominal injury while representing Australia at the ATP Cup.
"I have seen the physios, I have seen the doctors and I will be continuing to do that for the whole of the week and hopefully I will be ready to play in Melbourne," De Minaur said ahead of the Australian Open.
In Auckland, Italian Sinner – winner of the Next Generation ATP Finals in November – took number five seed Paire to a final set but the Frenchman eventually prevailed 6-4 2-6 6-4 in one hour and 55 minutes.
Defending champion Tennys Sandgren started his campaign with a routine 6-4 6-3 triumph over wildcard Michael Venus.
But last year's beaten finalist Cameron Norrie was knocked out in straight sets by qualifier Thiago Monteiro, with American Frances Tiafoe also being eliminated at the hands of qualifier Mikael Ymer.
Hubert Hurkacz and Ugo Humbert saw off Lorenzo Sonego and Casper Ruud respectively in Monday's other matches.
The 21-year-old has not played a match since a heavy loss to Jelena Ostapenko in Stuttgart in April. She subsequently withdrew from the Madrid Open and opted to undergo surgery on both wrists and one ankle.
Having initially targeted a comeback in late summer or early autumn, Raducanu has ended up missing the rest of the 2023 season.
Until recently there were doubts over whether she would make the start of next year but those have eased in recent weeks as she has stepped up her training.
And it has now been announced she will play at the WTA tournament in New Zealand beginning on January 1.
It will be Raducanu’s second appearance in Auckland and she will hope it is more positive than her debut in January, when she suffered an ankle injury during her second-round match and retired in tears.
The former US Open champion was able to recover to compete at the Australian Open but opted to undergo a procedure on her ankle to repair the damage in the spring.
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She criticised the “slippery” courts afterwards but has decided to accept a wild card to return.
Raducanu’s ranking has slipped to 296 and she faces a long road back to the top of the game, but it should give her the opportunity to fill in some of the steps she missed out thanks to her giant leap to stardom.
She can use a protected ranking of 103 to enter tournaments because of her long lay-off but that is currently not high enough to earn her a place in the main draw of the Australian Open.
Barring enough withdrawals of higher-ranked players, or a wild card, she will have to go through qualifying at a slam for the first time since her stunning title run in New York in 2021.
Reigning US Open champion Coco Gauff, former world number one Caroline Wozniacki and Wimbledon semi-finalist Elina Svitolina are among the other names confirmed for the Auckland tournament.
World number 12 Fognini was facing Feliciano Lopez, the oldest player in the draw at 38 who had already won a match earlier in the day as the tournament schedule caught up following weather delays.
Lopez beat Pablo Andujar in three close sets before coming from behind to prevail against Italian Fognini, winning 3-6 6-4 6-3 in just under two hours.
"The first match was tough for me, but I think it was very helpful because it was my first singles match of the season," said Lopez after his day of double duty.
"Without that match, I don't think I would have played the way I did against Fabio."
Australian John Millman recovered from a set down to oust Russian Khachanov 4-6 6-3 6-3.
Denis Shapovalov is the highest seed left in the draw, having defeated fellow Canadian Vasek Pospisil 6-4 7-6 (7-2).
Defending champion Tennys Sandgren is out after another former winner of the event, John Isner, triumphed 7-6 (7-3) 6-7 (1-7) 6-3 in an all-American round-of-16 encounter.
Seeds Benoit Paire and Hubert Hurkacz made it to the quarter-finals, as did Kyle Edmund and Ugo Humbert, who will now play Shapovalov.
At the Adelaide International, Felix Auger-Aliassime won his first match against wildcard James Duckworth, a 6-3 7-6 (7-0) win sending the second seed into the last eight.
Auger-Aliassime said: "Now I'm at a point where I was top 20 in the world. I reached three finals. Now every tournament I play, I play to win. I come, I want to reach the final. I want to give myself a chance to win titles."
Qatar Open champion Andrey Rublev continued his momentum with a 6-3 6-3 win over Sam Querrey and fourth seed Pablo Carreno Busta also progressed in straight sets.
There was a shock, though, as seventh seed Jan-Lennard Struff was beaten 6-3 6-4 by wildcard Alex Bolt.
Albert Ramos-Vinolas joined Bolt in the quarter-finals, as did qualifiers Lloyd Harris and Tommy Paul.
Norrie grew up in Auckland, and the British number one reached his first ATP Tour-level final in the city in 2019.
But despite repeating that feat and then winning the opening set of Saturday's final, Frenchman Gasquet roared back to lift his first title on the ATP Tour since 2018.
After they split the opening two sets, Norrie looked to be on the way to victory when he held a 4-1 lead in the decider.
But 36-year-old Gasquet rattled off five straight sets, including two breaks of serve, to shock the second seed and become the oldest champion in the Auckland Open's 66-year history.
"It's an amazing title for me, especially now at my age," Gasquet told a post-match news conference. "I really didn't think I would win again.
"I'm 37 this year, so when I came here last week, if you were to tell me next Saturday you will win here, I wouldn't believe it."
Norrie gets his Australian Open campaign underway against wildcard Luca van Assche on Monday, while Gasquet will play fellow Frenchman Ugo Humbert in the first round.
At the Adelaide International 2, Kwon Son-woo defeated Roberto Bautista Agut 6-4 3-6 7-6 (7-4) to take the crown.
Bautista Agut, who knocked out defending champion Thanasi Kokkinakis in the semi-finals, hit back from losing the opening set to level the game with a strong second stanza.
The deciding set went all the way to a tie-break as both players lost two of their service games, but with Kwon 5-4 up in the pivotal tie-break, the world number 84 found two breaks of serve to complete the victory.
The win was Kwon's second ATP Tour title and first since lifting the Astana Open trophy in 2021, while he becomes the first South Korean to win multiple Tour-level titles.
The 18-year-old was too good for Masarova throughout in Auckland, easing to a 6-1 6-1 victory in just an hour and 15 minutes on Sunday.
Gauff forced a break point in Masarova's first service game, and though the Spanish-Swiss player was able to rescue it, she was not so lucky in her next two as her ruthless opponent won the first set with back-to-back breaks.
That theme continued in the second as unforced errors from Masarova, who won just two out of 15 points on her second serve (13.3 per cent), saw Gauff break again immediately.
The American was then forced to save break points on her own serve but did so impressively as she rescued all 10 against her in the contest.
Masarova was able to get on the board again after saving another two break points, but that was as good as it got, with ultimately Gauff romping to victory in front of an impressed New Zealand crowd.
"It's been a great week for me, despite the rain," Gauff said, referring to tournament's frequent weather disruption, which also delayed the final and forced a pause at the end of the first set.
"It's my first title on hard [court] since I was 15, so I'm happy to do well on a surface that I love.
"It gives me a lot of confidence [ahead of the Australian Open]. You're never sure how your first week is going to go, but it's been a great week."
Gauff beat Kovinic 6-0 6-2 to move into her first hard-court final since claiming the title in Linz back in October 2019.
The top seed from the United States won the first eight games of a one-sided semi-final, making another statement just over a week before the Australian Open gets under way.
Gauff took only 73 minutes to dispatch seventh seed Kovinic and will face qualifier Rebeka Masarova in the final on Sunday.
Spaniard Masarova reached her first WTA Tour final courtesy of a 6-3 6-3 victory over Ysaline Bonaventure.
The 130-ranked Masarova, a junior French Open singles champion in 2016, served with assurance and struck 16 winners to break new ground.
It was also a memorable Saturday for Czech teenager Noskova, who claimed the scalp of world number two Jabeur with a 6-3 1-6 6-3 victory in Adelaide.
It was the 18-year-old Noskova's second win over a top-10 opponent this week, having also knocked out Daria Kasatkina.
Second seed Sabalenka will be a strong favourite to deny world number 102 Noskova the title on Sunday after she saw off Irina-Camelia Begu 6-3 6-2.
The Belgian lost both matches at the inaugural United Cup last week, but he saw off Bublik 6-3 6-4 in 74 minutes to reach the last 16 in New Zealand.
Another seeded player fell at the first hurdle on Monday as Adrian Mannarino lost 6-4 7-6 (7-4) to J.J. Wolf.
Richard Gasquet advanced with a 6-3 6-1 win over wild card Kiranpal Pannu, while Jenson Brooksby beat Fabio Fognini 6-7 (2-7) 6-1 6-3 and will now face Diego Schwartzman.
At the Adelaide International 2, fifth seed Dan Evans suffered a 7-5 7-5 defeat to Mackenzie McDonald.
Fellow seeds Alejandro Davidovich Fokina and Miomir Kecmanovic overcame Brandon Nakashima and Kyle Edmund respectively.
The British number one, who spent much of his childhood in New Zealand and Auckland specifically, reached his first ATP Tour-level final in the city in 2019, and has repeated that feat this time out.
Norrie made light work of Jenson Brooksby on Friday, winning 6-3 6-4.
He will face Richard Gasquet, who progressed via walkover due to Constant Lestienne's withdrawal through injury, in Saturday's showdown.
Norrie has won all six of his matches this season, three in Auckland and three at the United Cup, where he beat Rafael Nadal.
"It was an absolute battle with Jenson. A lot of long rallies and I know how well he competes, so it was nice to get it done in straight sets," said Norrie.
"He puts the ball in such awkward parts off the court and I had to come up with a lot of really tough shots on the run and a lot of big passes.
"I was able to serve it out and stay really calm and get over the line, but he's a great player."
At the Adelaide International 2, defending champion Thanasi Kokkinakis fell just short of reaching the final again.
He battled back from a set down to force a decider against Roberto Bautista Agut, but it was the Spaniard who prevailed 7-6 (7-4) 3-6 6-3.
Bautista Agut will face Soonwoo Kwon in the final, after the world number 84 defeated Norrie's compatriot Jack Draper 7-6 (8-6) 6-7 (2-7) 6-3.
The 18-year-old Czech was impressive against the third seed as she recorded her first win against anyone in the top 30 of the world rankings.
Fourth seed Veronika Kudermetova eased past Amanda Anisimova in just an hour and 15 minutes, winning 6-3 6-0, while Liudmila Samsonova will face second seed Aryna Sabalenka in the next round after coming from a set down to beat Zhang Shuai 5-7 6-3 6-0.
Irina-Camelia Begu and Elena Rybakina also both came from behind to secure wins, with the latter beating fifth seed Danielle Collins to set up a last-16 clash against Ukrainian Marta Kostyuk, who beat Jaimee Fourlis 6-1 3-6 6-4.
At the ASB Classic in Auckland, 42-year-old Venus Williams comfortably overcame her 21-year-old opponent Katie Volynets 7-6 (7-4) 6-2, and will play China's Zhu Lin next after she eliminated sixth seed Madison Brengle 4-6 7-6 (8-6) 6-4.
Third seed Leylah Fernandez beat Brenda Fruhvirtova 6-1 6-1 to set up a last-16 tie with Julia Grabher after she defeated Tereza Martincova in three sets.
Eighth seed Rebecca Marino is through after beating Dalma Galfi in straight sets and will now face Ysaline Bonaventure, who eventually put away Caty McNally 5-7 6-4 6-4.
Viktoria Kuzmova led fourth seed Bernarda Pera 5-4 in the opening set when rain stopped play for the day.
Raducanu was left in tears after retiring against Viktoria Kuzmova in their round-of-16 match at the ASB Classic.
The 20-year-old won the first set 6-0, but after losing the second set 7-5, Raducanu was unable to continue, sparking fears over her ability to play in the Australian Open in just 11 days.
But with her tournament opener against Tamara Korpatsch now just two days away, the 2021 US Open champion eased fears over her fitness, saying she "fully trusts" her ankle.
"I've been able to do preparation, albeit more limited than usual," Raducanu told reporters on Saturday. "But I'm feeling in a good place to go out there and give it my best shot.
"We were thrown a bit of a curveball but we remained optimistic. It's been a team effort to get me to this place. We've been building it up pretty gradually.
"For the ankle, I feel really good. It's going to be more introducing certain things, and the rate at which we've had to do it has been really quick.
"But I've not really played much tennis ever in my career, so I'm kind of used to it. And I'm not stressed about lack of tennis that much."
In Melbourne, Raducanu will be aiming to go beyond the second round of a grand slam for the first time since her incredible US Open heroics in 2021.
Raducanu had to come from a set down to beat Linda Fruhvirtova 4-6 6-4 6-2, while top seed Gauff had an easier time of it as she dismissed Tatjana Maria 6-4 6-1 on another day interrupted by the rain.
Second seed Sloane Stephens was only able to get through three games before play was suspended in her match against Rebeka Masarova, while fifth seed Wang Xiyu was 5-3 down in the first set against Karolina Muchova when the rain came.
Fourth seed Bernarda Pera is out after her match with Viktoria Kuzmova resumed having being washed out on Monday, and the American was unable to prevent a 6-4 6-4 defeat, with Kuzmova advancing to face Raducanu in the last 16.
The only two other matches completed on Tuesday saw Elena-Gabriela Ruse beat Erin Routliffe in three sets, and Anna Blinkova overcome Elisabetta Cocciaretto, also in three.
At the Adelaide International, Zheng Qinwen earned a hard-fought victory against number six seed Anett Kontaveit, eventually winning 6-1 4-6 7-6 (9-7).
Eighth seed Ekaterina Alexandrova also crashed out despite winning the first set against Marketa Vondrousova, going on to lose 4-6 6-3 6-2, though seventh seed Jelena Ostapenko is through after beating Karolina Pliskova 6-1 6-3.
In a contest that lasted over two-and-a-half hours despite only two sets being played, Victoria Azarenka beat Anhelina Kalinina 7-6 (11-9) 7-6 (7-5), and will face Zheng next, while Sorana Cirstea will go up against top seed Ons Jabeur in the last 16 after beating Viktorija Golubic in straight sets.
Raducanu dished out a bagel to qualifier Viktoria Kuzmova in the first set of the round of 16 match in Auckland on Thursday, but the 2021 US Open champion was unable to continue after losing the second set 7-5.
The 20-year-old Brit ended a disappointing 2022 season early due to a wrist injury and suffered another blow ahead of the first major of this year, leaving the court in tears.
Gauff will be eyeing her maiden grand slam title in 2023 and she impressively dispatched fellow American Kenin 6-4 6-4 in the second round in the capital of New Zealand.
Top seed Gauff and her compatriot were forced to play indoors due to the rain and it was the 18-year-old who prevailed, breaking three times and saving six of the break points she faced.
The teenager will face Zhu Lin of China for a place in the semi-finals after she beat Venus Williams 3-6 6-2 7-5.
Leylah Fernandez inflicted a crushing 6-0 6-1 defeat on Julia Grabher after losing only two games in her first-round win over Brenda Fruhvirtova and the Canadian will now face Ysaline Bonaventure, who defeated Rebecca Marino in three sets.
Top seed Ons Jabeur made a winning start to the season at the Adelaide International, seeing off Sorana Cirstea 7-6 (7-3) 6-1 to set up a meeting with Marta Kostyuk
Irina-Camelia Begu claimed the scalp of Jelena Ostapenko with a 6-3 6-0 upset of the seventh seed, while Victoria Azarenka was a 6-2 7-5 winner against Zheng Qinwen and teenager Linda Noskova got past Claire Liu in straight sets.
Ruud, who was beaten by Carlos Alcaraz in the US Open final last year, was the top seed in New Zealand but came unstuck in the round of 16 on Wednesday.
Djere came from behind to defeat the Norwegian world number three 3-6 6-3 7-6 (7-3) and seal his place in the quarter-finals, where he will meet Constant Lestienne.
In the process, Djere recorded his first career victory over a top-five opponent.
Ruud must now shrug off the disappointment and switch his focus to Melbourne, where he will be hoping to break his grand slam duck.
David Goffin overcame qualifier Christopher Eubanks in straight sets to tee up a last-eight tie with Richard Gasquet, while Jenson Brooksby received a walkover due to Diego Schwartzman's injury.
Unlike Ruud, second seed Cameron Norrie did make it through. The world number 12, who spent the majority of his childhood in New Zealand, enjoyed a homecoming as he saw off Jiri Lehecka 6-4 6-7 (4-7) 6-3.
Norrie is unbeaten in 2023, having beat Alex De Minaur, Taylor Fritz and Rafael Nadal while in action for Great Britain at the United Cup.
"Obviously, there were [some] nerves in me, coming back to New Zealand I wanted to play well in front of everyone, and Jiri's a great player," said the 27-year-old.
"So, it wasn't easy, and credit to him in that second set for playing a really good tie-break. He really took it to me, but I really enjoyed the time on the court."
There was also a shock at the Adelaide International 2, where top seed Andrey Rublev succumbed to wildcard Thanasi Kokkinakis.
Rublev hit back after losing the first set but Kokkinakis ultimately prevailed 6-4 3-6 6-3.
Second seed Pablo Carreno Busta also fell out, losing to Kwon Soon-woo, leaving third seed Karen Khachanov as the favourite after his straight sets defeat of Marc-Andrea Huesler.
Mikael Ymer, Jack Draper, Miomir Kecmanovic, Alejandro Davidovich Fokina and Roberto Bautista Agut all progressed.
The number two seed won 7-6 (10-8) 7-6 (7-3), saving seven set points after going 5-1 down in the first set, before roaring back to take both tie-breaks.
Sabalenka will face Marketa Vondrousova in the quarter-finals, with the Czech having produced a dominant performance in a 6-0 6-4 win against Kaia Kanepi.
"When it's 1-5 down, it's like you have nothing to lose, and you just go for your shots without thinking," Sabalenka said.
"That really helped me to stay in the set, to keep fighting and to keep trying."
Fourth seed Veronika Kudermetova eased through after a 6-4 6-0 victory against Bianca Andreescu, while Marta Kostyuk also advanced with a hard-fought 6-7 (5-7) 6-2 6-3 win over Elena Rybakina.
At the ASB Classic in Auckland, second seed Sloane Stephens is out after falling to Rebeka Masarova.
Having been forced to resume on Wednesday after rain suspended play the previous day, Stephens struggled against her Spanish opponent, eventually losing 6-3 7-6 (7-5).
Fifth seed Wang Xiyu is also out after retiring from her match against Karolina Muchova, but seventh seed Danka Kovinic is through after a simple 6-1 6-4 win against Nao Hibino.
Kovinic will face Lauren Davis next after she overcame Tamara Zidansek 6-2 6-1, while fellow American Sofia Kenin will take on number one seed Coco Gauff in the last 16 after she beat Wang Xinyu in straight sets.
Williams needed just 44 minutes to crush 18-year-old Anisimova 6-1 6-1 in the semi-finals of the WTA International event.
The 23-time grand slam champion was in fine form in the lead up to the Australian Open, producing 17 winners and six unforced errors to move into the final against Jessica Pegula.
"I was definitely in the zone today. It was just one of those days," Williams said during an on-court interview.
"I knew I was playing a really, really great player and an even better person so I knew I had to come out serious."
Williams added: "It feels good. I've been working really hard for a couple of years.
"My daughter is only two, I tend to be really hard on myself, but considering everything I'm doing pretty good."
Williams moved into a 98th WTA Tour final and will be aiming for a 73rd title and first since the 2017 Australian Open.
American superstar Williams and Denmark's former Australian Open champion Wozniacki have been firm friends for a long time, often spending time together away from the circuit.
But Wozniacki is just weeks away from retirement, so there was no time like the present for the pair to team up in doubles for the first time, and almost certainly the last.
And at the Auckland Classic, the crowd-pleasing Williams-Wozniacki partnership could be destined for silverware after a thumping 6-2 6-1 victory over top seeds Caroline Dolehide and Johanna Larsson took them through to the semi-finals.
"I'm happy to have my second win in 10 years in doubles! But I've got the best partner," Wozniacki said in an on-court interview.
Williams returned the compliment, saying, according to the New Zealand Herald: "We are having a blast. I'm having a blast. [She is] the most fun partner. It's great."
Former singles world number one Wozniacki has barely played doubles in the past decade, and had not won a tour match in the format since a rare outing alongside Malou Ejdesgaard at the 2011 Danish Open.
Williams, however, brings all the credentials in the world. When Williams plays doubles, invariably she is in it to win it.
She and sister Venus have a 100% record in the grand slam doubles finals they have reached as a team, landing 14 of those major titles together, while they also have picked up three Olympic titles.
Indeed, the last time Serena reached a women's doubles final but fell short was way back in 1999, when she and Venus were runners-up at the Southern California Open.
Wozniacki bows out of tennis after the Australian Open, having decided at the age of 29 the time has come to explore what else life has to offer.
She and Williams, who beat Nao Hibino and Makoto Ninomiya in round one, face a wait to learn who their last-four opponents will be.
But lurking on the other side of the draw, also into the semi-finals, are a pair who should play a big part in the future of women's tennis: American teenagers Coco Gauff and Caty McNally.
Gauff, 15, and McNally, 18, saw off the second seeds - Lara Arruabarrena and Renata Voracova - in their opening match and followed up by brushing aside Australians Jessica Moore and Arina Rodionova 6-3 6-2.
Williams and Wozniacki, playing as a pair for the first time, beat the top seeds in the quarter-finals and saved five set points in their opening set against Kirsten Flipkens and Alison Van Uytvanck before sealing a 7-6 (11-9) 6-2 victory to advance to the final.
They will play the winners of Saturday's all-American clash that sees Coco Gauff and Caty McNally take on Asia Muhammad and Taylor Townsend.
Earlier on Friday, Williams came from a break down in the second set to beat Laura Siegemund 6-4 6-3 and book an exciting semi-final against Amanda Anisimova, a 6-2 3-6 6-4 winner against wildcard Eugenie Bouchard.
Asked if she felt in good shape ahead of the Australian Open, Williams said: "I definitely feel that, and I feel like my match yesterday [against Christina McHale] was great.
"It's great that I went three sets, and it's great that I ran so much. I came out today just overall feeling better and doing better.
"Especially the match fitness – I'm super fit, but match fitness is a little different, so I think all these matches are really helping."
Wozniacki will face Jessica Pegula, who progressed when Alize Cornet retired at 6-0 3-2 down, after a routine 6-1 6-4 victory over defending champion Julia Goerges.
Osaka can improve her seeding ahead of the Australian Open if she makes the final at the Brisbane International, and standing in her way is her direct rival in world number two Pliskova.
Two-time major winner Osaka – ranked fourth in the world – had to fend off five break points to close out a 6-3 3-6 6-3 triumph over Kiki Bertens in the quarter-finals.
Reigning champion Pliskova lies in wait in the last four, though she had to stave off two set points in the first before winning 7-6 (8-6) 6-3 against Alison Riske. Petra Kvitova and Madison Keys will contest the other semi-final.
Garbine Muguruza was unable to kick off 2020 with a final appearance as she lost 6-4 6-3 to Ekaterina Alexandrova in the semi-finals of the Shenzhen Open.
Elena Rybakina, who beat Kristyna Pliskova 6-2 7-5, stands between Alexandrova and a first singles title on the WTA Tour.
Not since winning the 2017 Australian Open had Williams clinched silverware – the 23-time grand slam champion a runner-up at Wimbledon (2018 and 2019) and the US Open (2018 and 2019), while she retired in the 2019 Rogers Cup final.
However, Williams ended her drought by defeating fellow American Jessica Pegula 6-3 6-4 in the women's final in New Zealand.
"It feels good. It's been a long time. I think you could see the relief on my face," Williams said during her on-court interview before announcing she will donate her prize money (and a dress from each of her matches) to the bushfire relief effort in Australia.
Williams was almost flawless against Pegula, stepping up her preparations for the upcoming Australian Open with two aces and just one break of serve to capture her 73rd WTA crown – a record stretching across four decades after winning her first in 1999.
After a sluggish start, which saw the top seed fall 3-1 behind in the opening set, Williams eventually found her range en route to the 38-year-old's first trophy as a mother.
After snapping a sequence of five defeats in finals since her 2017 triumph at the Australian Open, Williams will look to equal Margaret Court's record of 24 slam titles at Melbourne Park.
Meanwhile, defending champion and second seed Karolina Pliskova won the Brisbane International for a second straight year after outlasting Madison Keys in three sets.
Pliskova prevailed 6-4 4-6 7-5 over eighth seed Keys for her third victory at the WTA Premier event in Brisbane.
