After counting down the Top 5 WTA tournament matches of 2018, our year-end review moves to the biggest upsets of the season. Up next is Naomi Osaka's composed win over 23-time Grand Slam champion Serena Williams in the 2018 US Open final, which marked a first major singles title for both Osaka and her country of Japan.

Top 5 WTA Upsets of 2018:
No.5: Zhang d. Stephens, Australian Open

No.4: Sabalenka d. Wozniacki, Montréal

WHAT HAPPENED: The last Grand Slam final of 2018 featured a tantalizing matchup between exemplars of the legendary present and spirited future of women's tennis, as 23-time Grand Slam champion Serena Williams, continuing her remarkable comeback to the WTA after a challenging pregnancy, met first-time Grand Slam finalist and highly-touted starlet Naomi Osaka.

Despite having roughly equivalent rankings at the time -- Williams was ranked No.26 to Osaka's No.19 -- the American superstar came into the final with an extremely overwhelming advantage in experience, having played 30 prior Grand Slam finals (eight having previously coming at the US Open), the most recent of which came at Wimbledon earlier in the summer.

Read the match report: Osaka stuns Serena, captures first Grand Slam title at US Open

By contrast, Osaka had never progressed beyond the fourth round of a Grand Slam event prior to her finalist run at Flushing Meadows. Moreover, the 20-year-old had only won one title before her US Open clash with Williams. That trophy, however, came earlier in 2018 at the Premier Mandatory event in Indian Wells, and Osaka directly followed that event up with a first-round win over Williams in Miami.

As the clash commenced, it became readily apparent that despite the gulf in big-match exposure between the two, Osaka was once again more than up to the challenge.

The Japanese rising star's glittering groundstrokes and sublime serve propelled her to a 5-1 lead before Williams was able to get her feet under her. Osaka won the first set 6-2 after getting 73 percent of service returns into play, blunting the vaunted Williams delivery.

WTA Insider Live Blog: Osaka lands first Slam, beats Serena in US Open 2018 final

Holding the one-set lead, Osaka then had to get over numerous hurdles in the remainder of her maiden Grand Slam final, which could have felled even the most experienced champion. Williams, one of the most notable comeback artists of all time, grasped the first break in the second set, to lead 3-1. Osaka, however, shook that off, using pristine composure to break back in the very next game.

Osaka's preternatural calm persisted throughout the rest of the deciding set, which contained rulings against Williams that amped up the already intense atmosphere. Osaka's mettle was tested, but she prevailed, breaking Williams to lead 5-3, and closing out the 6-2, 6-4 win with a service winner. The brilliant upstart had withstood all challenges to upset the six-time US Open titlist with aplomb.

WHAT THEY SAID: Osaka knew that at every turn, she had to maintain her composed demeanor. "I felt like I really had to focus during this match because she's such a great champion, and I know that she can come back from any point," Osaka admitted, during her post-match press conference.

Osaka claimed that "a different mindset" spurred on her US Open run. "I feel like coming into this tournament I had a lot of things happen to me, and then now I'm just having fun while I play, so I think that's always something that I can, like, keep trying to do."

Read more: 'When I hugged Serena, I felt like a kid again' - Osaka reflects on dream US Open win

Nevertheless, the subtext of the upset was that Osaka's career-best triumph came over her life-long heroine. "When I was growing up, I did a whole report on [Williams] in third grade," Osaka disclosed with a smile. "I colored it and everything. I said, 'I want to be like her.'"

"I know that, like, she really wanted to have the 24th Grand Slam," Osaka stated, tearing up in the process. "Everyone knows this. It's on the commercials, it's everywhere. Like, when I step onto the court, I feel like a different person, right? I'm not a Serena fan. I'm just a tennis player playing another tennis player. But then when I hugged her at the net...I felt like a little kid again."

As for Williams, the 36-year-old legend tried to keep her spirits up after the loss. "I'm just trying, like I said out there, to stay positive and to, you know, look at all the bright things and all the good things and just keep going forward with that."

Read more: Serena after US Open final: ‘I'm going to continue to fight for women’

"I always fight till the end and I always try to come back, no matter what," Williams continued. "But [Osaka] was also playing really, really well."

Williams piled on the praise for her young opponent. "She made a lot of shots. She was so focused. I think, you know, whenever I had a breakpoint, she came up with some great serve. Honestly, there's a lot I can learn from her from this match."

Serena Williams (L) hugs champion Naomi Osaka after the 2018 US Open final. (WTA/Jimmie48)

WHAT IT MEANT: Osaka's Grand Slam title was not only a personal achievement, but it was also a historic moment for Japan, as she became the first woman representing that country to win a Grand Slam title in singles. The victory also pushed her into the Top 10 of the WTA rankings for the first time, becoming the first Japanese woman to enter that echelon since Ai Sugiyama in 2014.

Osaka immediately looked forward to celebrating her Grand Slam triumph at the Toray Pan Pacific Open in Tokyo two weeks later. The Japanese player moved into the final there before Karolina Pliskova snapped her 10-match winning streak. Osaka would go on to finish the year in the Top 5 of the WTA rankings and qualified for the WTA Finals for the very first time.

Williams shut down her season after the US Open, but she ended the year as one of only two players (along with World No.1 Simona Halep) to reach two Grand Slam finals during 2018. It was a splendid comeback to top-tier tennis for the new mother, as she will continue to chase a record-tying 24th Grand Slam singles title in 2019.