After counting down the Top 5 matches at WTA events in 2017, our year-end review moves to the best Grand Slam matches of the season!

No.1 comes from the Big Apple itself, where Maria Sharapova pulled off a thrilling victory over No.2 seed Simona Halep under the lights of Arthur Ashe Stadium.

WHAT HAPPENED: All eyes were on the 2017 US Open women's singles draw as wildcard and former WTA World No.1 Maria Sharapova was set to play her first major tournament in just over 18 months. Most of those eyes promptly fell out of their heads when it was revealed that the 2006 champion would play No.2 seed Simona Halep, the woman in pole position to wrest the No.1 ranking from top seed Karolina Pliskova.

Halep was fresh off a run to the Western & Southern Open final, and though she'd never beaten Sharapova in six previous matches, three of the last four had gone the distance, including a dramatic French Open final clash back in 2014.

Read the match report: Sharapova shocks Halep in blockbuster US Open debut

Sharapova was decidedly below that level heading into the fortnight, having played just one match all summer after a left forearm injury forced her out of the Bank of the West Classic in Stanford.

For her part, the Romanian took the news as well as any top seed would.

"My reaction was normal, because this year I had very tough draws almost every time," she said during pre-tournament press. "Of course was a little bit, like, 'How is possible again?'"

The match was scheduled to follow the opening ceremonies on the first Monday of the tournament. Neither woman would shirk such a spotlight, and the pair combined to play one of the best matches of the season to the delight of the Arthur Ashe Stadium crowd.

Both Sharapova and Halep excelled on return to open the match, but the Russian claimed the three longest games to narrowly clinch the opening set, riding that momentum into a 4-1 advantage in the second.

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

Suddenly free of pressure, Halep reeled off the next five games, saving five break points in a thrilling tenth game to level the match at one set apiece

Just one break separated the two in the ensuing decider, with Sharapova breaking early and fending off a late break point to serve out the emotional win, 6-4, 4-6, 6-3.

"From the second I found out I was playing Simona - I was actually getting my nails done - I got my phone out and I pulled up YouTube videos of our matches and started studying our matches. We've just competed so well against each other in the past, really producing some good tennis."

- Maria Sharapova

WHAT THEY SAID: Sharapova is loath to let you see her sweat, but the veteran let the world in on just how much the victory - her 18th in a row in a US Open night session - meant after the two hour, 44 minute epic.

"Behind the Swarovski crystals and the little black dresses," she said, noting the glittering attire that helped her win WTA Fashion Fan Favorite, "this girl has a lot of grit, and she's not going anywhere."

The Russian later revealed the extent of that left forearm injury, one that forced her to miss Toronto, Cincinnati, and, just nearly, the US Open.

"Just looking back at August 12th, it was not a fun day: getting that MRI, getting the result of it, speaking to a doctor, flying to New York, getting another opinion, with the thought that I might miss the US Open.

"Looking back at that and seeing where I am today, it's just, I don't know, pretty amazing that I was able to produce that tennis because it wasn't there in the days leading up to this tournament."

Halep was understandably disappointed to end her Grand Slam season with a second loss in the opening round, even if it was under vastly different circumstances than the injury-addled defeat to Shelby Rogers at the Australian Open in January.

"I just kept fighting, playing," she explained in her post-match press conference. "I feel that I started to play a little bit more relaxed after [6-4, 4-1]. I didn't lose confidence during the match because I felt that I was very close, even in the first set.

"So it was a really good match, in my opinion. I mean, I cannot judge myself for not doing something, just the serve, because was a bad serve today."

It was indeed the serve that truly proved Halep's undoing; managing an average first serve speed of 89 miles per hour, it was the key difference in what was an otherwise stellar match off the ground.

WHAT IT MEANS: Sharapova went on to reach the second week at the US Open, getting within a set of the quarterfinals before fatigue set in against the always dangerous Anastasija Sevastova. Playing three more tournaments in 2017, she cemented her place in the Top 100 with a title at the Tianjin Open - the 36th of her career - assuring herself a spot in the Australian Open main draw next season.

Meanwhile, Halep headed back to the practice court and beefed up her serve, proudly displaying an improved delivery to score a first-ever win over Sharapova at the China Open. It was in Beijing where she finally clinched the No.1 ranking, becoming the first Romanian to take the top spot and hold onto it through the end of the year.