Naomi Osaka and Serena Williams set for a battle that could once again shift the power in women's tennis

• Serena Williams and Naomi Osaka are set for a blockbuster battle in the Australian Open semifinals on Thursday.
• The landscape of women's tennis shifted dramatically the last time the pair met at a Grand Slam.
• Now, Williams has a chance to reverse the power shift that took place just over two years ago, albeit against a much more assured Osaka this time around.

The last time Serena Williams and Naomi Osaka faced each other in a Grand Slam, the world looked very different than it does today.

The tennis world looked very different then, too.

In the lead-up to the 2018 US Open, Williams was on the upswing. Osaka was still a relative upstart.

After suffering complications while giving birth to her daughter, Olympia Ohanian, Williams delayed her highly-anticipated return to the court and extended her prolonged hiatus. Eventually, she was ready for her re-debut. And though the 23-time Grand Slam champion struggled in her first few tournaments, she finally began to resemble her former self as the year progressed.

Williams advanced to the Wimbledon final that summer, though she lost in straight sets to then-world No. 10 Angelique Kerber. Still, her run to the 30th Grand Slam final of her career gave fans the sense that Serena was ready once again to assume her rightful position atop the tennis throne and tie Margaret Court's record 24 career Grand Slam titles.

Even Williams herself felt the momentum shifting in her favor.

"These two weeks have really showed me that, 'Okay, I can compete,'" she said after that year's Wimbledon final. "Obviously, I can compete for the long run in a Grand Slam. I can, you know, come out and be a contender to win Grand Slams."

Osaka, meanwhile, headed into that year's US Open at just 20 years old. Though she'd already found some success, 2018 was already the best of her career by far. A series of firsts - including her first professional tournament title, first win against a current No. 1 player, and first top-25 world ranking - helped Osaka rise into the upper echelons of the tennis world. Still, she was a relative upstart and hardly a recognizable face to all except the most passionate tennis fans.

That is, until she shocked the world.

Osaka upset Williams in a swift and, ultimately, painful US Open final

Despite Williams' advantage in experience, power, and crowd favor, Osaka expertly outmaneuvered the six-time US Open champion from the start. She twice broke Williams' serve within the first 20 minutes of the match and cruised to a 6-2 first-set victory.

But before her dream of defeating her idol could become a reality, Osaka helplessly watched as controversy overshadowed her performance of a lifetime.

Osaka was well on her way to evening the second set at 1-1 when chair umpire Carlos Ramos interrupted the match's flow.

Ramos called a code violation against Williams after catching her coach, Patrick Mouratoglou, illegally motioning to Williams from inside her box. Williams was livid at the official's insinuation that she had cheated.

After Williams broke Osaka's serve to take a 3-1 advantage in the second set, she gave it right back upon hitting a sloppy backhand into the net. Williams slammed her racket onto the court in a moment of frustration, mangling the frame and earning an automatic code violation for the outburst. The second code violation gave Osaka a 15-0 advantage in the next game.

Williams, unsurprisingly, was not pleased, and after she exchanged words with the umpire, Osaka broke Williams' serve to take her first advantage of the second set at 4-3. Williams, even more heated than before, once again approached Ramos to push back against his rulings. But when Williams called him a "thief," Ramos issued a third code violation, which cost the superstar a full game and dug her into an even deeper hole.

Accusations of sexism, appeals to tournament authorities, and a held serve later, Osaka served for the match at five games to four in the second set. She maintained her composure enough to put the final nail in the coffin and complete the unfathomable upset.

Osaka's breakthrough had abruptly become a nightmare

As the crowd at Flushing Meadows booed vociferously, and both parties approached the net, the 20-year-old first-time Grand Slam champion pulled down her visor to hide her tears. She hugged Williams, shook hands with Ramos, then returned to her bench to sob into her hands.

The trophy ceremony was no less brutal. Fans at Arthur Ashe Stadium picked up their booing, and Osaka once again pulled her visor over her face to shield her crying. Williams cried, too, but she reached over to comfort her much-younger opponent and later encouraged the crowd to stop booing.

Osaka wasn't in the proper frame of mind to issue a proper acceptance speech. Instead, she deferred the interviewer's question and apologized directly to the crowd.

"I know that everyone was cheering for her," she said into the microphone. "I'm sorry it had to end like this. I just want to say thank you for watching the match."

Osaka's big win shifted the landscape of women's tennis for years to come

Everything has changed for Osaka in the two and a half years since that fateful US Open match. After abruptly catapulting into the limelight in Queens, the Japanese international held onto her newfound fame through a combination of on-court success and undeniable likability.

In a fourth-month span, she went from having never played in a Grand Slam final to becoming a two-time Grand Slam champion. She earned her first world No. 1 ranking shortly after that, then quickly became one of the most marketable - and wealthiest - athletes on the planet.

In one fell swoop, the naturally quiet and understated Osaka found her voice en route to a third career Grand Slam victory. And almost as quickly as she had emerged on the tennis scene, Osaka had become the face of women's tennis.

"People often forget how young the athletes they follow are," Osaka told Insider earlier this month. "When I started, I was still somewhat of a kid, finding my way both on the court and in the world around me. I feel that finding your place in the world also requires years living in the world."

Williams, meanwhile, still has yet to break through to earn her highly-coveted 24th Grand Slam title. She advanced to both the Wimbledon and US Open finals in 2019, but she lost in straight sets at both tournaments. Injuries have plagued her in the years since her US Open defeat to Osaka. Though Williams returned to the top-10 of the world tennis rankings, fans are still anxiously awaiting the return of "vintage Serena."

There's an overwhelming sense that the queen is back.

Osaka has won 19 consecutive matches heading into Thursday's blockbuster Australian Open semifinal match. Through power, grace, and precision, she has staved off defeat for an entire calendar year. No one would be blamed for calling Osaka the frontrunner this time around, even as she faces off against her childhood idol and the undisputed greatest competitor in the sport's history.

Williams appears to be as close to peak form as she's been since her 2017 maternity leave. She was clinical in her dismantling of world No. 2 Simona Halep on Monday, besting the star in straight sets en route to the grudge match against Osaka.

And though Williams has repeatedly expressed her respect for and admiration of the young superstar, she undoubtedly has something to prove against her on Thursday. The match provides Williams with an opportunity to finally set the record straight and play a high-profile match against Osaka without the controversy and distraction that clouded their 2018 championship battle. It's a chance for Williams to reassert her dominance, to remind the world who runs the sport, and to reverse the power shift that took place two and a half years ago.

But this time around, the Osaka standing across the net is not a shy, softspoken newcomer with everything to gain and nothing to lose. She's a bona fide titan of women's tennis, the heir apparent to the sport's throne, and the greatest threat to Williams' bid to secure her legacy once and for all with Grand Slam No. 24.