To mark the end of a record-setting 2019 season, is counting down our picks for the best WTA Upsets of the year. Check out our Top 5 Grand Slam Matches and our Top 5 WTA Matches here.

Coming in as the No.5 WTA Upset of 2019 is Sofia Kenin's second-round win over World No.1 Ashleigh Barty at the Rogers Cup, which was the American's first-ever Top 5 victory and kicked off a run of form which pushed her into the Top 20.

WHAT HAPPENED: Ashleigh Barty came into the Rogers Cup nearly literally on top of the world, as the Australian entered the event in Toronto holding the World No.1 ranking after winning titles at Roland Garros and Birmingham. 

A stumble against Alison Riske in the fourth round of Wimbledon did not knock Barty off the summit of the WTA singles rankings before the tour moved into the meat of the summer hardcourt season in August.

Barty, though, had a challenging opponent for her first match on hardcourt since April, as she followed up a first-round bye with a clash against 20-year-old Sofia Kenin of the United States. The rising American had climbed into the Top 30 after her first three WTA singles finals in 2019, converting two of those finals into titles, at Hobart and in Mallorca.

Kenin had opened her Toronto campaign with a first-round triumph over wily veteran Hsieh Su-wei, which set her up for a second-round encounter with top-ranked Barty. Like Hsieh, Barty carves her way through matches with crafty play, but also relies on strong serves as an addition to her winning equation.

Despite her methodical rise into the Top 30, Kenin had yet to claim a Top 5 win at that juncture, having gone 0-3 against that cohort. Kenin had also lost all three of her previous meetings with Barty.

Therefore, one could surmise that grasping a win over the newly-crowned World No.1, who had won 15 of her last 16 matches, including a maiden Grand Slam title in Paris, would still be a difficult task for the young American to pull off.

2019 Toronto highlights: Kenin bests Barty

When the duo started their tilt, Kenin went up an early break at 2-1 in the opening frame, and served for a one-set lead at 5-4. However, Barty fired an error-forcing forehand to break the American and level the set at 5-5, then ramped up the aggression on her serve and groundstrokes to eke out a tough tiebreak, putting the Australian a set away from yet another match-win.

The feisty American, though, continued to effectively scamper around the court, cracking a backhand crosscourt service return for a clean winner and a love service break as she clinched a commanding 4-0 lead in the second set. Barty reeled off three straight games to get back on serve, but Kenin regained her advantage, polishing off the set with a forehand winner.

Kenin kept her momentum going in the opening stages of the decider, firing a passing winner to go up 4-1 and once again obtain a double-break advantage. Barty clawed one of the breaks back, but Kenin kept her nose in front, using a forehand winner to reach 5-3, as she edged closer to a first Top 5 win.

Barty fought off three match points on her serve in the next game, forcing Kenin to serve for the match at 5-4, but the ever-steely American did not blink, and reached a fourth match point on her service. There, Barty fired a forehand long, and Kenin had knocked the World No.1 out of the Rogers Cup 6-7(5), 6-3, 6-4, in just under two hours.

The upset was notable not just because Kenin’s first Top 5 win came over a World No.1 who was on a roll. Adding to that accomplishment, Kenin claimed the mark as the youngest American to defeat a current World No.1 in eight years, and became just the second American aged 20 or younger to beat a World No.1 in the past 18 years.

Read the match report: Kenin battles past Barty in Toronto to score first No.1 win

WHAT THEY SAID: "I went into the court and tried to already get adjusted to the game, because [Barty] plays very differently to other players,” Kenin said to WTA Insider, after the signature victory. “That's why she's No.1.”

“The first set didn't go my way, but I was playing against the World No.1, so of course she was going to come up with amazing serves, start playing better and pick up her game,” the victor continued. "I just had to rewind, forget about it and just battle out there."

“I usually don't start off so well, and usually the first set goes by pretty fast. It takes me a while to find my rhythm against her," Kenin added, while reflecting upon her first win in four meetings against Barty.

"Whenever you play her, Ash is going to give everything and luckily, it just went my way today," the American stated.

Barty, now on a two-match losing streak, was nevertheless magnanimous in defeat. “You need to be confident and trust yourself and really hit through the ball, and today I didn't have quite enough of that,” the top seed said, during her post-match press conference. “And against a player like Sofia, who hits the ball so cleanly and so aggressive and takes it so early, that's something you need.”

“It wasn't there today, but she played an exceptional match when she needed to, and in the big moments she played some really good stuff,” Barty concluded.

WHAT IT MEANT: After knocking out Barty, Kenin stated that "my confidence level is high. I'm playing well and having a good year. I love the big stages. I love the big matches. That's why I play...just being there, that's why I work hard.”

Indeed, Kenin proved over the next fortnight that her confidence and big-match expertise were at new peaks, as she would use her first-ever victory over a World No.1 to propel herself to an excellent run over the two-week period encompassing the Premier 5 events in Toronto and Cincinnati.

First, the American continued her superb performance at the Rogers Cup with another Top 10 win over Elina Svitolina, before falling to eventual champion Bianca Andreescu in the semifinals.

The next week at the Western & Southern Open in Cincinnati, Kenin pulled off the feat all over again, but with a twist. Kenin beat Svitolina once more to reach the Cincinnati quarterfinals, where she claimed her second victory over a World No.1 in two weeks.

This time around, though, that World No.1 win came over a different player -- Naomi Osaka, who had supplanted Barty at the top spot after Toronto. Osaka retired while down 0-2 in the third set of their encounter.

In Cincinnati, Kenin once again lost to the eventual titlist in the final four -- this time, it was Madison Keys, who grasped a closely contested straight-set win. However, after posting back-to-back semifinal showings at consecutive Premier 5 events, the 20-year-old made her Top 20 debut after Cincinnati, on August 19.

Kenin finished off a breakthrough season with a third WTA singles title in Guangzhou in September, and peaked at World No.12 before finishing the season ranked World No.14 -- a 38-spot jump from her 2018 year-end ranking of No.52. As a result, Kenin finds herself as a nominee and heavy contender for this season’s WTA Most Improved Player of the Year award.

As for her vanquished opponent, despite the loss and a resultant dip from the top spot of the WTA Rankings, Barty still went on to easily post the finest season of her career, culminating in a title at the Shiseido WTA Finals Shenzhen and rebounding to complete 2019 with the year-end World No.1 ranking. Along the way, Barty avenged her loss to Kenin, beating the American in Wuhan.