Osaka's title defence continued with a 6-2 6-4 win over Saisai Zheng in the second round at Melbourne Park on Wednesday.
Seeded third for this year's opening grand slam but ranked fourth in the world, Osaka was dethroned as the WTA's number one by Ash Barty following the US Open.
When it was put to her that she could return to the top of the rankings with back-to-back Australian Open titles, Osaka - who has points to defend - looked on quizzically.
"Get through where to be number one in the world?," the confused Japanese star replied to the interviewer.
Top seed Barty has 8,017 points, well ahead of Osaka's 5,496.
Pressed again on the possibility of returning to the summit, Osaka said: "Oh really?
"No I thought because Ash [Barty] is like 2,000 points ahead of me and I have to defend this, that I'm going to drop out of the top 10 for real. OK that's good news for me... I guess."
Osaka overcame warm but breezy conditions to advance to the third round of the slam in Melbourne, not before her frustrations spilled over in the second set.
After dropping serve, Osaka dropped her racquet and kicked it before storming through to the next round.
"I definitely got very frustrated in the second set, and it's something that I knew would happen, but I didn't know the scale, like, what she would do to make me frustrated," Osaka told reporters in her post-match news conference.
"I think that I just really have to focus on knowing that I'm going to have really tough matches and I have to find a way to go around it and just be consistently level-headed."
Osaka added: "She was slicing and dicing and getting everything. I was, like, 'Can I just hit a winner already?' She was, like, 'No, I'm going to get everything'. That's basically what happened."
In warm but breezy conditions in Melbourne, third seed Osaka defeated China's Zheng 6-2 6-4 on Margaret Court Arena on Wednesday.
Japanese star Osaka will next face either teenage sensation Coco Gauff or Sorana Cirstea at Melbourne Park.
Osaka made a strong start in front of a healthy Japanese contingent, breaking in the second game for a 2-0 lead, only to be broken back immediately after firing a forehand long.
Zheng managed to get back on serve, however, consecutive double faults handed the break back to Osaka, whose power and precision helped wrap up the set in 34 minutes.
Osaka fended off two break points in the opening game of the second set, but Zheng continued to have her chances on the two-time grand slam champion's serve.
Zheng, who also saved a break point to hold for 1-1, finally broke at the fifth time of asking as she moved ahead 2-1.
The disciplined approach from Zheng frustrated Osaka, who threw and kicked her racquet before claiming the break back instantly. The pair traded breaks as Osaka's unforced errors grew, until the latter composed herself to see out the match after one hour, 20 minutes.
STATISTICAL BREAKDOWN
Naomi Osaka [3] bt Saisai Zheng 6-2 6-4
WINNERS/UNFORCED ERRORS
Osaka – 20/30
Zheng – 7/20
ACES/DOUBLE FAULTS
Osaka – 0/2
Zheng – 0/3
BREAK POINTS WON
Osaka – 6/12
Zheng – 3/7
FIRST SERVE PERCENTAGE
Osaka – 59
Zheng – 68
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Osaka – 63/50
Zheng – 50/29
TOTAL POINTS
Osaka – 61
Zheng – 46
Barty – who last featured in a clay-court match en route to Roland Garros glory almost two years ago amid the coronavirus pandemic – made light work of Misaki Doi 6-2 6-1.
Miami Open champion last week, top seed Barty continued where she left off at the WTA 500 event on Wednesday, closing out proceedings in just over an hour in Charleston.
"I had to use it as an opportunity to try and build some comfort," Barty said. "I think having a couple days between Miami and playing this first match was important, to have my body rest, but to also try and prepare to change surfaces. It was a very quick turnaround, but I felt great out there tonight."
Barty – who now boasts a 15-2 win-loss record in 2021 – will next face Shelby Rogers after she outlasted 13th seed Amanda Anisimova 1-6 7-5 6-4.
As for second seed Sofia Kenin, the 2020 Australian Open champion slumped to a shock 4-6 6-3 6-4 defeat against Lauren Davis midweek.
World number four Kenin – runner-up at last year's French Open – eased to the first set but fellow American Davis, ranked 79th in the world, changed tactics to fine effect.
"I was a bit uncomfortable so I just really dug deep and really started to think about how to beat her and I found that heavy deep balls were not her favourite," the 27-year-old Davis said.
"Being on clay, the ball bounces high, pushing girls back, so it's very effective."
Kenin took a medical timeout at 3-0 down in the deciding set but, despite reeling off the next three games, she was unable to see Davis off as her 2021 woes continue.
Davis now heads into another all-American clash with teenage sensation Coco Gauff – who came from 1-3 down in the third to defeat Liudmila Samsonova 4-6 6-1 6-4 in an absorbing two-hour contest.
Belinda Bencic – the fifth seed – crashed out as Paula Badosa claimed the first top-20 win of her career following a 6-2 6-7 (2-7) 6-1 success.
Badosa will face Caty McNally in the last 16 after the 19-year-old American beat Anastasija Sevastova 7-6 (7-3) 2-6 6-4.
Seventh seed Elise Mertens was another big name to depart as Alize Cornet prevailed 7-5 6-3, although 15th seed Veronika Kudermetova and 12th seed Ons Jabeur progressed in with the minimum of fuss.
Former US Open champion Sloane Stephens, meanwhile, topped eighth seed Madison Keys 6-4 6-4.
Seeds were even more vulnerable at the Copa Colsanitas in Bogata, including the top-seeded Saisai Zheng who was dispatched 6-4 6-1 by Stefanie Voegele.
Zheng's compatriot Yafan Wang, seeded eighth, went down 6-3 6-3 to Viktoriya Tomova.
Arantxa Rus and Jasmine Paolini, the third and sixth seeds respectively, lost in three to Spanish duo Nuria Parrizas Diaz and Lara Arruabarrena.
World number one Barty needed three sets to see off Australian Open finalist Muguruza in Doha, while Kvitova won a pair of tie-breaks in her quarter-final with Jabeur.
Veteran Svetlana Kuznetsova earned a share of tournament history with an upset win over fourth seed Belinda Bencic.
The Russian now faces Aryna Sabalenka, who came from behind to defeat Saisai Zheng in the day's opening match.
BARTY OUT TO CONTINUE KVITOVA RUN
Barty endured a second-set dip in her 6-1 6-7 (4-7) 6-2 beating of Muguruza, who has reached at least the last eight in each of her five tournaments in 2020.
The Australian, playing at her first event since reaching the semis of her home slam in Melbourne, did not face a single break point in the opening set but was met with a much sterner test in the second.
Barty raced into a 5-2 lead in the decider, though, and fought back from 40-15 in the final game to set up a showdown with Kvitova, who she has beaten in their previous four meetings - including a straight-sets win at the Australian Open.
Kvitova, the 2018 champion, edged to a 7-6 (7-2) 7-6 (7-4) win over Jabeur, who has enjoyed a stellar year thus far and became the first Arab woman to reach a major quarter-final in Melbourne.
MILESTONE FOR KUZNETSOVA
Kuznetsova was in devastating form against Bencic, hitting 39 winners and just 15 unforced errors in a 6-4 6-4 triumph.
A runner-up in 2004 and 2007, Kuznetsova now has 19 match wins in Doha, which ties the record held by retired former world number one Caroline Wozniacki.
"I have amazing memories," said Kuznetsova, who is in to her first Doha semi since 2007.
"It was my first time when I beat Justine [Henin] here [in the 2004 semi-finals] and it was like I was a kid, I was 19.
"I climbed from number 30-something to number 13 or number 14 and it was amazing. It was so long ago it was like another life, but it really happened to me - I didn't watch it in the movies."
Sabalenka will try to end Kuznetsova's run after a 3-6 7-6 (7-2) 6-3 defeat of Zheng.
The American wrapped up a 7-6 (7-5) 6-1 victory and will now face top seed Barty, who will still be smarting from her surprise quarter-final exit at the Australian Open.
Barty is the reigning champion in Adelaide and will be seeking to put that loss to Karolina Muchova firmly behind her.
Collins' compatriot Shelby Rogers also prevailed, beating Veronika Kudermetova 6-3 7-6 (7-4) to set up a clash with third seed Johanna Konta.
Anastasija Sevastova overcame Caroline Garcia 6-2 6-4, while eighth seed Wang Qiang was a 6-4 6-3 winner over teenager Olivia Gadecki.
Second seed Osaka was playing just her second match on clay since the 2019 French Open due to injury issues and became the latest big name to fall at the hands of Muchova.
The world number 20, who had already defeated two top-five players this season, prevailed 6-4 3-6 6-1 in a time of one hour and 49 minutes in the Spanish capital.
Osaka has now lost two of her last three matches, having been eliminated from the Miami Open quarter-finals by Maria Sakkari at the end of March.
"I think today for me it was quite different from the last time I lost in Miami, and I actually think I played much better here, so I'm very happy about that," Osaka said.
"I think what I can take away from this is that I tried my best throughout the entire match. I think tennis-wise, I felt I was too defensive in the first set.
"I wouldn't say I’m happy that I lost, but I think I learned a lot. So that's all I can hope for."
Two-time champion Halep has yet to drop a set in this year's tournament after seeing off unseeded opponent Zheng 6-0 6-4.
Halep took nine games in a row at the start of the match but, like in her opening-round win over Sara Sorribes Tormo, she had to hold off a late surge from her opponent to advance.
Next up for Halep is a last-16 showdown with Elise Mertens, who beat Elena Rybakina 7-6 (7-4) 7-5 in the last of Sunday's matches.
Fifth seed Aryna Sabalenka safely advanced earlier in the day thanks to a 6-3 6-3 win against Daria Kasatkina, while Sakkari beat Anett Kontaveit 6-3 6-1.
Sabalenka was on course to face Victoria Azarenka for a place in the quarter-finals, but the Belarussian withdrew from her match with Jessica Pegula because of a back injury.
Elsewhere, Jennifer Brady, who knocked out fellow American Venus Williams in the first round, eased past Jelena Ostapenko 6-2 6-1.
Brady faces a tricky test in the next round with Anastasia Pavlyuchenkova, who defeated sixth seed Karolina Pliskova 6-0 7-5, seeking another scalp.
Li triumphed 7-5 6-1 over third seed Zheng, who held the highest rank of all remaining players after Sloane Stephens and Nadia Podoroska's first-round exits.
World number 52 Zheng struggled to win on her first serve, with Li converted five of seven break points.
American Li, who is ranked 72nd in the world, will next face Swiss qualifier Viktorija Golubic, who beat Russia's Anna Kalinskaya 6-2 6-4.
On the other side of the draw, Canadian Leylah Fernandez defeated Slovakia's Viktoria Kuzmova 7-5 6-3 and will face seventh seed Sara Sorribes Tormo in the other semi-final.
Sorribes Tormo, who is now the highest ranked remaining player, defeated Anna Karolina Schmiedlova 6-3 7-5.
Zheng, who is ranked 27th in the world, triumphed 6-1 6-2 in a dominant display where she was seldom challenged.
In the last eight, Zheng will face eighth seed Ann Li, who overcame Slovenian Tamara Zidansek 6-4 6-3 at the WTA International tournament.
Zheng is the highest remaining seed left standing in the draw after the first-round exits of top seed Sloane Stephens and Nadia Podoroska.
Russian qualifier Anna Kalinskaya continued her good run with a 6-4 6-3 victory over Nina Stojanovic and she will next face Viktorija Golubic in the quarters – the latter topped Lauren Davis 6-3 6-1.
