After counting down the Top 5 WTA matches and Top 5 Grand Slam matches of 2017, our year-end review moves to the biggest upsets of the season!

No.5 on the countdown is big-hitting Naomi Osaka’s methodical takedown of US Open defending champion Angelique Kerber.

WHAT HAPPENED: Japan’s 19-year-old wunderkind Naomi Osaka once again made her mark and, this time, she did it on the sport’s biggest stage.

No.45-ranked Osaka put together a brutal display of power tennis to defeat US Open defending champion Angelique Kerber in the first round, striking 22 winners to Kerber’s nine and needing just 65 minutes to complete the 6-3, 6-1 upset.

No US Open defending champion had lost in the first round the following year since Svetlana Kuznetsova in 2005, but Kerber's service woes gave big-hitting Osaka the in that she needed.

Read the match report: Kerber suffers shock first-round defeat to Osaka

She swung freely against the too-passive Kerber, drawing first blood at 5-3 in the first set and powering ahead 3-1 in the second. The Japanese teen kept her composure as her lead grew, putting any memories of last year’s collapse behind her to seal the victory - and the first Top 10 win of her career - after just one match point.

WHAT THEY SAID: The last time Naomi Osaka played on Arthur Ashe Stadium, the teenager dissolved into tears as her commanding 5-1 final set lead over Madison Keys dissipated. One year on, she finally laid the ghost of that loss to rest.

"This court hasn't really been a fond memory, but now I hope I changed that,” Osaka said in her post-match interview. “At 4-1 I thought, ‘I really hope I don't do what I did last year.’

"[At the end] I felt really relieved, especially since I was so nervous on the last point. I just barely returned the serve. I just really didn't want to play a long point on the last one, so I was really glad when she made an error."

She added, "I was really happy because I grew up watching the greatest players play on that court, so to win a match on it felt really special.”

WHAT IT MEANS: It wasn't just a milestone win for Osaka: the victory also marked the highest-ranked scalp for a Japanese player since Akiko Morigami upset World No.3 Nadia Petrova at Roland Garros in 2006.

For Kerber, in addition to the lopsided loss the German also lost a whopping 2,000 ranking points. As a result, exactly one year after ascending to No.1 for the first time Kerber would fall out of the Top 10 for the first time since 2015 after the US Open fortnight.

But despite exorcising her Arthur Ashe demons against Kerber, Osaka couldn’t go on to replicate the result. The German would get her double revenge in Asia, defeating Osaka in the opening rounds of the China Open and Toray Pan Pacific Open.

Read more: Top 5 Grand Slam 2017 Matches:
No.5: Konta vs. Vekic - Wimbledon
No.4: Vandeweghe vs. Bouchard - Australian Open 

No.3: Halep vs. Svitolina - French Open
No.2: Muguruza vs. Kerber - Wimbledon
No.1: Sharapova vs Halep - US Open