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

Top 5 Best WTA Matches of 2019:
No.5: Simona Halep d. Bianca Andreescu, Shiseido WTA Finals Shenzhen
No.4: Ashleigh Barty d. Petra Kvitova, Miami Open
No.3: Belinda Bencic d. Naomi Osaka, Mutua Madrid Open
No.2: Aryna Sabalenka d. Caroline Wozniacki, Nature Valley International
No.1: Naomi Osaka d. Bianca Andreescu, China Open

Coming in at No.1, Naomi Osaka came from a set down, and a break down in the deciding set, to get her rivalry with Bianca Andreescu off to a winning start in the year's first match between reigning Grand Slam champions.

2019 Beijing highlights: Osaka halts Andreescu

WHAT HAPPENED: There were only three matches across this year's WTA Tour season between reigning Grand Slam champions, and the first of them came as late as October, when Australian Open victor Naomi Osaka and US Open winner Bianca Andreescu clashed in the Beijing quarterfinals.

That wasn't the only reason the match was a must-watch blockbuster, though. It was the first meeting between two of the game's most charismatic young stars, polar opposites in many ways, pitting the thunderous power of the introverted Osaka against the all-court flair of Andreescu, whose game is as extra as her heart-on-sleeve personality. Little wonder than many observers felt it could be the start of an era-defining rivalry.

Read the match report: 'I'm still here' - Osaka comeback halts Andreescu winning streak in Beijing quarterfinal thriller

As to that, only the future will tell - but this time, at least, the clash lived up to every bit of the hype. It was dramatic, full of momentum shifts and on a knife-edge to the very end as both players sought to wrest control from each other: Andreescu leapt out to a 5-1 lead, Osaka pegged her back to 5-5, but the Canadian snuck out the first set anyway. A close second set found Osaka making the first breakthrough and raising her level to sustain her lead to level the match, but the Japanese player needed to overturn a 1-3 deficit to edge a thrilling dénouement on her third match point. It was suitably epic, clocking in at two hours and 14 minutes; and, most importantly, it was high-quality, with the level of both players rising as it went on - a testament to their competitive spirit.

Over the course of an up-and-down year, Osaka has been open about her insecurities and doubts as she has sought to live up to the expectations that come with being a two-time Grand Slam champion and World No.1.The 22-year-old's willingness to unpack her feelings in public is both admirable and relatable - but, as both this match and her remarkably similar win over Petra Kvitova that topped wtatennis.com's countdown of this year's best Grand Slam matches demonstrate, this should never be mistaken for mental weakness. In both high-profile matches, with a statement win on the line, Osaka was able to ride out the twists and turns of the scoreboard to snatch victory via - when it really came down to it - the more clutch player on court. She may not pretend it's easy, but Naomi Osaka has nerves of steel.

WHAT THEY SAID: "I don't want to play her any more!" joked Osaka after battling for over two hours. "I'm good, one and done."

Expanding, the former World No.1 explained just how awkward Andreescu's game had been for her to play. "Of course I've watched her play on TV, but it's so different from actually playing against her," she said. "I know that she is incredibly smart. She knows when the rally isn't working out for her, when to change it up and make it difficult for the other person. So just to experience that in person was very frustrating."

Osaka also admitted that she had been unable to ignore the buzz around the match. "It affected me - it just made me really nervous," she revealed. "In the beginning we were both scoping each other out. She was probably thinking, like, Wow, what is she doing? I'm just hitting so many unforced errors. She's like, Wow, she won two Grand Slams like that? I was just trying to settle in the first set. I could not find the mental line of not being nervous and also being fired up."

Read more: Naomi Osaka hits back, preaches perspective after Stuttgart run

Beneath the tension, though, Osaka had something to prove. "[The win] meant a lot because I feel like people counted me out after the Europe thing," she asserted (Osaka did not reach a final on clay or grass, nor did she reach the second week of either Roland Garros or Wimbledon). "I still won a slam this year, I won Tokyo. I'm still here."

Joking aside, Osaka knows that both she and Andreescu will still be here for many years to come - "It's bound to happen again" - and the success of younger rivals serves as motivation. "I like seeing the younger players," she mused. "I really love seeing Coco [Gauff], Iga [Swiatek], her do well. It makes me think like they're younger than me so I should be able to do the things they're accomplishing."

Andreescu was in agreement. ""I think we're going to have many matches like this," predicted the 19-year-old. "Our game styles are pretty different, but they level up pretty equally. If she's playing well, like she did, I play well, then I think they level up very similarly."

For the Canadian, the result also brought a 17-match winning streak to an end; indeed, she had not lost a completed match since Sofia Kenin won their Acapulco semifinal at the start of March. "I forgot how it feels," Andreescu admitted. "Honestly, it sucks. I didn't miss it at all. But at least I didn't get whooped 1 and 1. I put on a fight. Honestly, it could have went either way. It was just some points here and there. At the same time I am pissed, but at the same time I'm proud of myself with how I played today. I really fought, especially in that last game. Holy crap, that was crazy!"

WHAT IT MEANT: Osaka had described Andreescu as the "hottest player on tour" - but in ending the teenager's winning streak, she was extending her own. Osaka would go on to lift the Beijing trophy, defeating another reigning Grand Slam champion in the final in the shape of Roland Garros victor Ashleigh Barty - the first time in her career that she had won back-to-back titles - and her own 11-match winning streak is still alive heading into 2020.

Sadly, though, there would be no rematch at the WTA Finals in Shenzhen to close the year. Both Osaka and Andreescu would be struck down by injury in the round-robin stages: Osaka's right shoulder forced her to withdraw after one match, a three-set defeat of Petra Kvitova, while Andreescu sustained a knee injury that forced her to retire during her second match against Karolina Pliskova.