After a day that included mystery gong sounds, butterflies and three thrilling matches that pushed the action into the early hours of the morning, the semifinal lineup at the 2016 U.S. Open has been set. While the top seeds in both the men’s and the women’s event remain in the hunt, there have been significant upsets and storylines elsewhere. Here’s what to look for in the four semifinals.

Serena Williams vs. Karolina Pliskova (Thursday, 7 p.m. EDT, ESPN)

Having come into the final Grand Slam of the tennis year with doubts over a shoulder injury, Serena Williams has answered those questions with an emphatic serving display through to the last four. In five matches, the American, who is on a quest to land a an Open-era record 23rd Grand Slam title in New York, served up 60 aces and only faced one break point before a tough three-set encounter with Simona Halep in the quarterfinals.

Indeed, Thursday’s first semifinal should feature a plethora of aces, with Williams’ opponent Karolina Pliskova ranking third on the tournament list with 32. That’s just one less than second-placed Ana Konjuh, who the Czech beat in straight sets to secure a place in her first Grand Slam semifinal. While Pliskova also knocked out Venus Williams en route through, Serena’s experience and greater movement and weapons gives her a significant edge.

Prediction: Williams win

Serena Williams Serena Williams overcame Simona Halep in a high-quality three-set U.S. Open quarterfinal on Wednesday. Photo: Getty Images

Caroline Wozniacki vs. Angelique Kerber (following the first semifinal, ESPN)

The only expectations on former world No. 1 Caroline Wozniacki entering the U.S. Open came from herself. Hampered by injury, the Dane came in with a ranking of 74 and a 13-14 record in 2016. But the two-time finalist at the U.S. Open has rediscovered her form in a city where she lives for at least part of the year. Wozniacki has dropped just one set, in the first round, and recorded impressive wins over Svetlana Kuznetsova and Madison Keys.

She faces an arduous task, though, to make it to a third final at Flushing Meadows. Angelique Kerber has already reached two Grand Slam finals in 2016, beating Serena Williams in Australia before the American got revenge at Wimbledon. Having arrived at the U.S. Open on the back of winning the title in Cincinnati, the German has yet to drop a set in New York. And, while she has shared her past four matches with Wozniacki, Kerber is a different player this year and her greater aggression allied with a solid defense should see her take care of business on Thursday.

Prediction: Kerber win

Novak Djokovic vs. Gael Monfils (likely the first men’s semifinal on Friday at 3 p.m. EDT, ESPN)

In a tournament during which he celebrated his 30th birthday, Gael Monfils has been all business, shedding, at least in part, the showman tag that has been used as much to deride the Frenchman as it has to praise him during his career. Monfils has reached his first U.S. Open semifinal without dropping a single set, although he has faced just one seed and that was a physically spent Lucas Pouille. Certainly Monfils will know that it is a very different challenge awaiting him in the semifinals.

Novak Djokovic has won all 12 of the pair’s previous meetings, dating all the way back to the first round of the U.S. Open when the pair were still teenagers. While Monfils’ path through to the final four has been smooth, Djokovic has barely had to break sweat. Injuries to three of his opponents mean that the world No. 1 has remarkably completed only nine sets in New York. Although there is a case to be made that questions over a wrist and arm injury have yet to be fully answered, the Serbian was impressive in the two sets he completed against Jo-Wilfried Tsonga in the quarterfinals.

Prediction: Djokovic win

Stan Wawrinka vs. Kei Nishikori (likely following the first semifinal on Friday, ESPN)

One would hope that Stan Wawrinka and Kei Nishikori would be handed the later semifinal on Friday after both came through grueling quarterfinals on Wednesday. Nishikori went first and caused the biggest upset of the tournament to date, beating world No. 2 and the form player this summer, Andy Murray. Displaying a mental and physical resolve that haven’t always be present during the 27-year-old’s career, Nishikori struck back from a two-sets-to-one deficit and held his nerve as Murray faltered amid the distractions of intrusive stadium noises and insects under the roof on Arthur Ashe.

Nishikori is now looking to repeat his run to the U.S. Open final two years ago, during which he overcame Wawrinka in five sets in the quarterfinals. Wawrinka needed just the four sets to get through his quarterfinal this time around, although it was still a punishing encounter with Juan Martin del Potro that didn’t finish until past 1 a.m. in New York. To get the job done, the Swiss produced his best form of what had previously been a largely underwhelming showing in New York. There is always the chance that Wawrinka rediscovers the barraging play that earned him two Grand Slam titles, but, after his win over Murray and a straight-sets victory over Wawrinka just over a month ago in Canada, Nishikori would appear to have the edge.

Prediction: Nishikori win