We've already witnessed plenty of surprises so far in the US Open. In the round of 16, both the 2018 men's and women's singles champions, Novak Djokovic and Naomi Osaka, lost out on their bid to defend their titles. More top players, like Simona Halep (second round), Ashleigh Barty (round of 16), Kei Nishikori (third round) and Roger Federer (quarterfinals) also saw early exits.

Both the men's and women's semifinals will be set after the conclusion of Wednesday's quarterfinal matches. Here's how to watch those. Until then, here are eight takeaways from the first week and a half of play at the USTA Billie Jean King National Tennis Center in Flushing Meadows:

Serve and volley play making a comeback

It's been a long time since the women's game has seen a player use the serve and volley to make noise in a major championship. The game today is baseline-focused, and servers are rarely seen moving quickly towards the net.

But the serve and volley style was brought front and center in Arthe Ashe Stadium during American qualifier Taylor Townsend's three-set win over Simona Halep in the second round. In the match against Halep, Townsend caught the 2019 Wimbledon champion completely off guard with her aggressive charges forward to the net.

Through the first three rounds of Townsend's play at the Open, she served and volleyed on 45 percent of her service points, winning 71 of her 126 attempts. Townsend not only proved her strategy to be effective, but she pleased the US Open crowds with her play.

Williams has her best chance for No. 24

No. 8 seed Serena Williams advanced to her 10th US Open final on Thursday with a straight sets victory over No. 5 Elina Svitolina. Williams, a 23-time Grand Slam champion, is the only woman remaining in the singles field to have won a major title. Williams has only played one three-set match during her road to the semifinals, and she has a winning record versus the remaining players left on the women's field. It's her best chance, and obviously the final chance she'll have this year, to complete her quest for the record-tying 24th Grand Slam championship. A Grand Slam title this year would also be Williams' first major championship since becoming a mother in 2017. 

Federer-Nadal championship won't happen

The US Open won't see a Roger Federer-Rafael Nadal championship this year. It's an obvious takeaway, but one worth reflecting on. Federer, 38, experienced upper back and neck issues during his five-set quarterfinal loss to Grigor Dimitrov. Federer was expected to have no problems reaching the semifinals after having a 7-0 record against Dimitrov in their previous meetings. With Djokovic out (who Federer lost to in an epic Wimbledon final), this was Federer's best opportunity to get his 21st Grand Slam title. Nadal, 33, will face Matteo Berrettini in their semifinal match on Friday night. It's a real bummer for tennis fans, who were hoping to see the two add to their storied rivalry and face off at the US Open for the first time. Now, with two of the Big Three out of this year's tournament, the door's wide open for Nadal to get his 19th Grand Slam title and close in on Federer for the all-time record. Djokovic isn't far behind with a total of 16 Grand Slam titles.

Kerber, Stephens close out disappointing seasons

Two of the top players on the women's professional circuit, Angelique Kerber and Sloane Stephens, closed out disappointing seasons after early exits at the US Open. Kerber's season concluded with a three-set loss in the first round to Kristina Mladenovic, while Stephens lost in straight sets to Anna Kalinskaya in the first round. Kerber also saw early exits in the earlier grand slams, bowing out in the second round at both the French Open and Wimbledon, and fourth round at the Australian Open. Stephens also saw a fourth round exit at the Australian Open, and lost in the quarterfinals at the French Open and third round at Wimbledon.

Kerber, 31, saw her best season in 2016 when she won the Australian Open and the US Open. She also won Wimbledon in 2018. Stephens, 26, won the US Open in 2017. Both Kerber and Stephens have undergone coaching changes this season, but neither was able to reach their full potential in 2019. Kerber's been without a coach on tour since July after she parted with Rainer Schuttler, while Stephens dismissed Sven Groeneveld and reunited with Kamau Murray, who was her coach when she won the Open in 2017.

American women on the upswing

This year's US Open showed positive signs for the future of American women's tennis. We've already witnessed the rise of American players Madison Keys, Sloane Stephens, Amanda Anisimova and Sofia Kenin, but now we have a few, new names to keep tabs on. Kristie Ahn and Taylor Townsend both were unexpected players to reach the round of 16, but with their success, the pair made it 10 different American women to have reached the Round of 16 or better at a Grand Slam event in 2019: 

  • Serena Williams (QF - Aus. Open; F, Wimbledon; R16+, US Open)
  • Sloane Stephens (R16, Aus. Open; QF, French Open)
  • Madison Keys (R16, Aus. Open; QF, French Open; R16+, US Open)
  • Danielle Collins (SF, Aus. Open)
  • Amanda Anisimova (R16, Aus. Open; SF, French Open)
  • Alison Riske (QF, Wimbledon)
  • Coco Gauff (R16, Wimbledon)
  • Sofia Kenin (R16, French Open)
  • Kristie Ahn (R16+, US Open)
  • Taylor Townsend (R16+, US Open)

Djokovic's shoulder injury

Novak Djokovic was forced to retire in his round of 16 match against Stan Wawrinka because of pain in his left shoulder. He was clearly in pain throughout the match, and after the loss, told reporters that he was battling through the pain in his shoulder during his first three round matches at the Open. While the seriousness of the injury is unknown, hopefully Djokovic will take some time to rest and recover from the injury, especially since when the 19-time Grand Slam winner played through an elbow injury in 2017-18, it resulted in surgery and a long hiatus from tennis.