To mark the end of a record-setting 2019 season, wtatennis.com is counting down our picks for the best matches of the year.

Top 5 Grand Slam Matches of 2019:
No.5: Barty d. Anisimova, Roland Garros
No.4: Andreescu d. Mertens, US Open
No.3: Pliskova d. Hsieh, Wimbledon

Continuing the countdown in the Grand Slam category at No.2 is Serena Williams's defeat of No.1 seed Simona Halep after going toe-to-toe in the fourth round of the Australian Open.

WHAT HAPPENED: Coming into the first major of the year, both Simona Halep and Serena Williams were somewhat unknown quantities. Halep was the World No.1, but she had lost her first match back after a three-month hiatus due to a back injury in Sydney to Ashleigh Barty. Williams, meanwhile, had shut her 2018 season down early following her dramatic loss to Naomi Osaka in the US Open final.

Yet both seemed to be rounding into something approaching their best form as they booked their fourth-round clash in Melbourne. Halep, the previous year's runner-up to Caroline Wozniacki, had been taken to three sets twice - though given that this was by renowned upset artist Kaia Kanepi and freshly crowned Hobart champion Sofia Kenin, who would be one of 2019's breakout young talents, there was no shame in that; the Romanian completed her navigation through a tough first week by defeating Venus Williams 6-2, 6-3.

Serena, by contrast, had been imperious in extending her winning streak at a tournament she had won while eight weeks pregnant on her last appearance in 2017, needing 70 minutes or less to dispatch each of Tatjana Maria, Eugenie Bouchard and Dayana Yastremska. It was this form that the American, continuing her quest for a 24th Grand Slam title, took into her first meeting with a World No.1 in six years. Smacking 10 winners past a stunned Halep, Williams needed only 20 minutes to take a one-sided opening set 6-1.

Read the match report: Serena seals Halep victory, rounds out Aussie Open QFs

But down a set and a break, facing a point to fall behind 1-3 in the second set, Halep's tenacity kicked in. Taking more risks and directing the ball from line to line, the 2018 Roland Garros champion saved that point with a smash - and suddenly, a contest that had been a rout transformed into a classic as two of the game's biggest names went toe to toe for one hour and 47 minutes.
As Halep began to carve up the court with precise geometry, doggedly extending rallies and testing Williams's movement, she forced the match on to her territory. The middle period of the match saw Halep capture eight out of 11 games, with a bold backhand winner down the line setting up a set point - duly and efficiently taken - to level the match as her 1-2 second-set deficit turned into a 3-2 final-set lead.

This time it was Williams's turn to fight back. The champion summoned up some of her most aggressive and accurate tennis to stave off three break points - two with unreturnable serves, naturally - before a backhand winner down the line of her own sealed the hold. As is so often the case, missed opportunities would immediately backfire for Halep: the very next game saw a resurgent Williams press home her momentum, breaking Halep for the first time since early in the second set with a forehand winner. This would be decisive as the only break in the third set: Williams dropped only two more points behind her serve as she closed out her sixth straight victory over Halep, and ninth in 10 meetings overall.

WHAT THEY SAID: Williams treated the victory as a learning experience in a comeback that, at this point, was still less than a year old. Satisfied with her initial level - "I played really good in that" - the 23-time Grand Slam champion said: "That goes to show you that you have to play well for two sets. You can't just play well for one set or six games. You have to bring it every single point, every single game, until the match is literally over."

The reason, of course, was an opponent who hasd been undaunted by getting blown away in the first set. "That's why she's No.1," said Williams admiringly. "She literally lifted her game to a new level. I didn't. I kind of stayed at the same level, and I should have looked at my game, as well. But it's a part of this journey on my way back... it's 10 months, so I can't be too upset at myself. I felt like I did have an opportunity to win that in straight sets, but then I'm playing the No.1 player in the world."

Halep's assessment tallied with Williams's. Though she joked that "I felt like I had been hit by a train in the first set", Halep shrugged off the idea that she might have been negatively affected by it. "I didn't get scared about the first set, because I knew I have a better level," she pointed out. "I can play better if I stay there and I really start moving better and hitting the ball stronger. So after the first set, I got fire inside myself, and I said that now I start the match. So it was much better."

Despite the loss, Halep gave herself "close to 10" for her Melbourne performance overall, taking into account her layoff prior. "I have the courage to say that, because I took the risk to stay home so much, and I haven't been prepared for the highest level in tennis," she said. "But I did not play bad, so I'm happy about the way that it's been going this tournament, and I take only the positives. I had great matches. I had the toughest draw, but it was nice."

WHAT IT MEANT: Strangely enough, while this early heavyweight clash reaffirmed the quality of the tennis both players were capable of - as though anyone needed reminding - it did not end up setting the tone for either the tournament or the year. Williams's supreme form was halted in the next round in one of the strangest losses of her career as she squandered a 5-1, 40-0 lead and four match points against Karolina Pliskova - the most match points she had let slip in a loss since Monica Seles had saved six to win their 2001 Los Angeles quarterfinal. A jarred ankle as that match slipped away from her would be the start of several months of injury woes - and although Williams has found the level she delivered in this match on several occasions in the second half of the year, both a 24th major and a first post-maternity title remain elusive.

Halep, meanwhile - having declared 2018 a "year of chill" after finally achieving her long-awaited dream of a Grand Slam title - would spend the first half of 2019 posting similar results: intermittently excellent tennis, several creditable battles, but no titles to show for it. After the Australian Open, she would lose her World No.1 ranking to Naomi Osaka; and, after her Roland Garros title defence was halted unceremoniously in the quarterfinals by a 17-year-old Amanda Anisimova, Halep slid out of the Top 5 for the first time since May 2017. But she would turn her year around - and avenge this loss - in the most unexpected of fashions at Wimbledon. Facing Williams again in the final, with the weight of the head-to-head against her, Halep put the memory of all the contests in which she had fought hard but come up short behind her - bypassing the battle for the blowout as she stormed to her second major title 6-2, 6-2.