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 Aryna Sabalenka's first Top 5 victory at the Coupe Rogers, stunning Australian Open champion Caroline Wozniacki to kick start her career in Canada.

WHAT HAPPENED: It was déjà vu all over again for Aryna Sabalenka, who was up against former World No.1 Caroline Wozniacki for the second time in three months at the Coupe Rogers in Montréal. The pair last played in the final of the Nature Valley International in Eastbourne, where the Belarusian enjoyed a banner week alongside new coach Dmitry Tursunov en route to her first Premier-level final.

That loss sent Sabalenka on a three-match losing streak that lasted until the Mubadala Silicon Valley Classic in San Jose, leaving the youngster ready for revenge. Saving three match points, she hit a stunning 64 winners to outlast the Dane, 5-7, 6-2, 7-6(4), and earn the first "biggest win" of her career.

Read the match report: Surging Sabalenka wraps up Wozniacki comeback in Montréal

Wozniacki had struggled most of the summer with various injuries - including one that forced her out of the Citi Open just a week prior - yet looked solid to start her rain-delayed clash with Sabalenka, winning the final four games of the opening set.

The 20-year-old's power soon took its toll as she leveled the match and kept pressing even as the 2017 BNP Paribas WTA Finals Singapore presented by SC Global champion edged closer to victory in the decider.

Sabalenka saved three match points from 3-5 down in the final set - all on her serve - and promptly broke the next game, ultimately saving her most dominant display for the tie-break two win after two hours and 31 minutes on court.

WHAT THEY SAID: The day wasn't over for Sabalenka, who had to play a second match of the day against Elise Mertens due to the rain-wrecked schedule, but the Belarusian still managed her off-beat sense of humor in between.

"This was a good warm-up for me," she joked, adding, "I had to serve well, because I had the chance to serve for the set in Eastbourne.

"That didn't work well in the first set, so I was a little disappointed, but I tried to keep staying in the game, taking my serves, and tried to be quiet. I did it, finally, so I feel so happy."

Wozniacki was equal and opposite in her reaction, visibly disappointed about the missed opportunities late in the match.

"I feel like I would have felt after losing in the finals," she said, pointing to her opponent's relentless aggression as what turned things arond.

WHAT IT MEANT: Though Sabalenka would later fall to Mertens that same day - ironically from match point up - that whirlwind day in Montréal was just the start as she rolled into her first Premier 5 semifinal a week later at the Western & Southern Open.

Read more: Starstruck Sabalenka adjusts to rising profile at US Open

A week after that, she captured her first WTA title at the Connecticut Open, and headed into the US Open as many a pundit's darkhorse pick to pull off her biggest surprise yet at the USTA Billie Jean King National Tennis Center.

"Everything is the same," she insisted at the time. "The players are treating me like normal, like nothing has changed. I like this, because if they started treating me differently, it would make me wonder, ‘What’s going on?’ But they’ve been the same, like always. I won a title, but I’m the same person. Why do I have to be something different?"

Playing Naomi Osaka for a spot in the quarterfinals, she narrowly fell to the eventual champion, but shook off that defeat to win her first Premier 5 title at the Dongfeng Motor Wuhan Open, just missing out on a chance to make her WTA Finals debut in Singapore.

As for Wozniacki, physical struggles got worse before they got better, forcing her out of Cincinnati and haunting her through most of the Asian Swing before she turned around to win her first Premier Mandatory title since 2011 at the China Open.

The former No.1 used the win as a springboard to play more solid tennis in Singapore, getting within two games of the semifinals before bowing out to future champion Elina Svitolina in her final round robin match.

In her final press conference of the year, she admitted to dealing with rheumatoid arthritis, having been diagnosed with the illness since the US Open.