The 2019 Miami Open champion defeated Sabalenka 6-4 6-7 (5-7) 6-3 in humid conditions in South Florida to reach the last four, where the Australian will face fifth seed Elina Svitolina.
Svitolina won in the later match on Tuesday over Latvia's Anastasija Sevastova 6-3 6-2.
Barty, the 2019 French Open champion, did not drop her serve once against the Belarusian, saving seven break points throughout the match as she bids to retain her top ranking.
Barty is under threat from 2021 Australian Open winner Naomi Osaka for the number one ranking, with the Japanese also into the quarters.
The Australian will retain the top ranking by reaching the decider, regardless of second seed Osaka's results.
"It was a brilliant match today," Barty said in her on-court interview. "It was certainly the best level I’ve played for a long time and especially over a sustained period.
"Aryna really made me bring out my best stuff today. She’s an exceptional competitor."
Barty has now won 10 consecutive matches in Miami, dating back to her 2019 title with 2020's event cancelled due to the COVID-19 pandemic.
Ukraine's Svitolina, ranked fifth in the world, reached the Miami Open semi-finals for the first time with her win over the unseeded Sevastova.
"I've always wanted to play well here in Miami," said Svitolina, who made last year's French Open quarter-finals.
"It's one of the biggest tournaments and playing here is always amazing, so I'm really pleased with the performance tonight."
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.
The Italian had already beaten defending champion Karolina Pliskova en route to the last eight and pulled off another upset as she claimed a 7-6 (7-5) 0-6 6-4 win over Sabalenka.
Giorgi's bid for the third WTA Tour title of her career will now continue with a semi-final against Anett Kontaveit on Friday.
The Estonian also needed three sets to book her place, bouncing back from losing the opener to beat Viktorija Golubic 2-6 7-6 (7-2) 7-5.
Daria Kasatkina was unable to build on her win over fourth seed Iga Swiatek in the last round as she missed out on a spot in the semi-finals.
The Russian took the first set against Jelena Ostapenko but failed to build on that success as the Latvian ran out a 1-6 7-5 6-2 winner.
In the final match of the day, Anastasija Sevastova was beaten by Elena Rybakina, who had earned her place in the quarters by beating second seed Elina Svitolina the day before.
World number 61 Sevastova, who eliminated Coco Gauff, won the first set 6-2, but Rybakina fought back to claim the next two 7-6 (9-7) 7-6 (7-5).
Rainy conditions ensured there was no play at this week's other WTA event held in Bad Homburg, Germany, though one player was able to secure passage to the semi-finals.
Sara Sorribes Tormo claimed her place in the next round after Victoria Azarenka, her scheduled opponent, pulled out due to injury.
In her first professional match in almost half a year, world number two Halep, who skipped last week's Palermo Open and will not make a decision on whether to play the US Open or not until after the tournament in the Czech Republic, was made to work hard for victory by the unseeded Hercog.
The Romanian picked up an early break and responded to dropping her serve in game three of the first by reeling off the next four in succession to take the set.
When Halep started the second set by breaking Hercog the writing appeared to be on the wall, but the Slovenian world number 46 fought back valiantly to force a decider in emphatic fashion.
Halep was unable to convert any of the six match points that came her way after breaking for a 5-3 lead, with Hercog clinging on to take the match to a third-set tie-break.
After failing to consolidate an opening mini-break, Hercog's resistance was finally broken after two hours and five minutes when a forehand into the net sent top seed Halep into the second round.
Wildcard Barbora Krejcikova ensured Halep's next match would not be an all-Romanian affair by beating Patricia Maria Tig 6-4 6-3.
Second seed Petra Martic defeated Varvara Gracheva 7-6 (7-2) 6-3 and third seed Elise Mertens needed three sets to overcome Jasmine Paolini 7-5 4-6 6-3.
A number of other seeded players were not as successful. Ekaterina Alexandrova (5) fell to qualifier Lesia Tsurenko, while Anastasia Pavlyuchenkova (6) went down 7-5 6-2 to Arantxa Rus, and Anastasija Sevastova (9) lost 6-2 6-2 to Irina-Camelia Begu.
Halep beat Caroline Garcia in the second round in Florida on Thursday, but an issue with the world number three's right shoulder prevented her from facing Anastasija Sevastova.
The two-time grand slam champion said: "I'm very sorry to I have to pull out of the singles and doubles at the Miami Open, but my injury doesn't let me play here as expected.
"I'm sad that I can't continue, I wanted to come here to give my best and play many matches but unfortunately I can't.
"Hopefully next year I will come back healthy and better."
Sevastova will face either French Open champion Iga Swiatek or Ana Konjuh in the round of 16.
Barty faced a match point but the defending champion and world number one dug deep to prevail in three sets against Kristina Kucova on Thursday.
Romanian third seed Halep also needed three sets at the WTA Premier tournament in Miami.
BARTY PRODUCES COMEBACK
In her first match outside of Australia in more than 12 months, Barty survived to top qualifier Kucova in Miami.
Barty trailed 5-2 in the third set, with Kucova earning a match point at 5-3 but the Australian star saved it and won the final five games of the match to secure a spot in the round of 32.
Winner of the 2019 French Open, Barty sent down an equal career-high 15 aces in two-and-a-half hours.
"Today was really hard work, and I enjoyed every single minute of it," Barty said on court post-game. "There’s nothing like coming through a test like that, and now I get another opportunity in a couple days' time to play another tough match and test myself again."
Next up for Barty is 2017 Roland Garros champion Jelena Ostapenko, who outlasted Kirsten Flipkens 6-2 5-7 6-3.
HALEP REACHES MILESTONE
Halep brought up her 400th WTA main-draw triumph with a 3-6 6-4 6-0 victory over France's Caroline Garcia.
After dropping the opening set, Halep won 11 of the final 12 games to move through to the next round.
Halep won in one hour, 48 minutes, drastically improving on her first serve after the opening set while coming to grips with Garcia's kick serve.
"I needed time to get used to it and to get the rhythm," Halep said about Garcia's serve having improved her head-to-head record to 7-1, with Anastasija Sevastova awaiting in the last 32 after eliminating American sensation Coco Gauff.
Halep received attention on her shoulder early in the second. "I struggled with my serve," Halep said. "I struggled with my shoulder a little bit, so the attention was a little bit on the pain. [After that] I started to relax myself."
KERBER'S DOUBLE BAGEL
Three-time grand slam winner Angelique Kerber has been up and down in recently, but she flexed her muscles with a 6-0 6-0 humiliation of Renata Zarazua.
Aryna Sabalenka – the seventh seed – was another one to fend off match points before rallying 0-6 6-3 7-6 (11-9) past Bulgarian qualifier Tsvetana Pironkova in a gutsy.
Last year's French Open champion Iga Swiatek knocked off Barbora Krejcikova 6-4 6-2, while ninth seed and two-time Wimbledon champion Petra Kvitova got past Aliza Cornet 6-0 6-4.
Fifth seed Elina Svitolina survived to beat Shelby Rogers 3-6 7-5 6-3 and two-time major winner Victoria Azarenka benefited from a walkover.
The world number five lost 6-4 7-6 (7-3) to 21st-ranked Rybakina, whose reward is a meeting with Coco Gauff's conqueror Anastasija Sevastova in the quarter-finals.
Third seed Bianca Andreescu of Canada also lost in straight sets, Anett Kontaveit of Estonia beating her 6-3 6-3.
And it was the same story for fourth seed Iga Swiatek, though she at least took a set as she was beaten by Daria Kasatkina.
It took an impressive rally from Russia's Kasatkina to bounce back from losing the first en route to a 4-6 6-0 6-1 success.
She will now face Jelena Ostapenko, who beat Ons Jabeur 5-7 6-4 6-3.
In fact, top seed Aryna Sabalenka was the only seed to avoid a surprise exit on Wednesday as she cruised through.
The Belarussian, ranked fourth in the world, beat Alison Riske 6-1 6-4 in just over an hour to set up a clash with Camila Giorgi.
Giorgi had earlier followed up a win over defending champion Karolina Pliskova in the last round by beating Shelby Rogers 6-3 4-6 6-2.
There was no such string of shocks at Wednesday's other WTA event, the Bad Homburg Open in Germany, though first and second seeds Petra Kvitova and Victoria Azarenka needed three sets to progress.
Kvitova lost a second-set tie-break as she beat Ann Li 6-2 6-7 (4-7) 6-1 to reach the quarters, while Azarenka defeated Alize Cornet 6-4 3-6 7-6 (9-7).
Angelique Kerber, the fourth seed, progressed more smoothly, earning a comfortable 6-0 6-2 win over Russia's Anna Blinkova.
And Nadia Podoroska saw off the challenge of Patricia Maria Tig, winning 7-6 (7-4) 2-6 6-4.
Gauff, 16, fell in the opening round of a grand slam for the first time after a 6-3 5-7 6-4 loss to Latvian 31st seed Anastasija Sevastova.
Teenager Gauff thrilled the tennis world last year when she made the fourth round at Wimbledon, before repeating that feat at this year's Australian Open.
However, there will be no such memorable run at Flushing Meadows after a loss to Sevastova that lasted just over two hours.
Gauff finished with 40 unforced errors, including 13 double faults, to fall in the opening round.
She could have faced Naomi Osaka in the third round once more, the Japanese star having thrashed Gauff at the same stage in a memorable clash last year.
