The 2018 U.S. Open quarterfinals continued in Flushing, New York, on Wednesday, with the remaining spots in the semifinals on the line looking to be filled.

Top-seeded Rafael Nadal and Juan Martin del Potro previously locked up their spots on the men's side, with Serena Williams and Anastasija Sevastova doing the same in the women's draw. As a result, four players battled for two spots on Wednesday.

Below is a look at how the action unfolded.

2018 U.S. Open Quarterfinals Results — Wednesday

Men's Singles

No. 21 Kei Nishikori def. No. 7 Marin Cilic, 2-6, 6-4, 7-6 (5), 4-6, 6-4

No. 6 Novak Djokovic def. John Millman, 6-3, 6-4, 6-4

*Full draw available on the U.S. Open's official website

Women's Singles

No. 20 Naomi Osaka def. Lesia Tsurenko, 6-1, 6-1

No. 14 Madison Keys def. No. 30 Carla Suarez Navarro, 6-4, 6-3

*Full draw available on the U.S. Open's official website

Women's Singles

No. 20 Naomi Osaka def. Lesia Tsurenko, 6-1, 6-1

Naomi Osaka had cruised through much of the 2018 U.S. Open through this point—and it was more of the same in the semifinals as she looks to win her first career major.

Osaka had swept her way through the first three rounds, having shut her opponents out in three consecutive sets heading into the fourth round. She was tested against No. 26 Aryna Sabalenka on Monday, ultimately prevailing, 6-3, 2-6, 6-4. 

This match was more reminiscent of her first three matches.

Osaka dropped just two games in her straight-set victory over Lesia Tsurenko on Thursday, a match that took less than an hour to play. And the numbers, via the tournament's official website, tell the story.

She won 91 percent (20 of 22) of her first-serve points and won 58 percent (29 of 50) receiving points. Not only that, but Tsurenko committed 31 unforced errors while her opponent had just 11. That proved to be a recipe for success for Osaka on this day.

With the victory, the 20-year-old Osaka now advances to her first career Grand Slam semifinals. Her previous best result at a major tournament was a fourth-place finish at the Australian Open back in January. According to the U.S. Open, Osaka is the first Japanese woman to reach the semifinals since 1996.

Osaka now awaits Madison Keys in the semifinals, which will be played on Thursday.

No. 14 Madison Keys def. No. 30 Carla Suarez Navarro, 6-4, 6-3

Like Osaka before her, Madison Keys has barely had to break a sweat throughout the tournament thus far. While Carla Suarez Navarro didn't make things easy, Keys once again advanced with a straight-set victory.

The first set was a tight battle between the two ranked players. They posted nearly identical numbers in winners (8-8), first-serve points won (Keys 75, Navarro 72) and second-serve points won (Keys 47, Navarro 42). Despite committing six additional unforced errors, Keys managed to grab the first set, 6-4.

The second set was Keys' all the way. She won an impressive 86 percent of her first-serve points, which helped her cruise to a 6-3 set.

With the victory, Keys has put herself in position to try to join an exclusive club, as ESPN Stats and Info points out:

This marks Keys' second consecutive trip to the semis, as she lost to Sloane Stephens in the finals last year. The 23-year-old does not have a major title in her career.

Keys will now face Naomi Osaka the semifinals, which will be played on Thursday.

Men's Singles

No. 21 Kei Nishikori def. No. 7 Marin Cilic, 2-6, 6-4, 7-6 (5), 4-6, 6-4

With a five-set victory over Marin Cilic, Kei Nishikori earned a spot in the semifinals after prevailing in a hard-fought battle.

Cilic came out strong by taking the first set with relative ease, 6-2:

However, Nishikori was able to respond by winning each of the next two sets:

That put Cilic on the brink of elimination, which he had an answer for. After a back-and-forth battle for four sets, this showdown needed a fifth set to decide the winner.

Ultimately, it was Nishikori who claimed victory.

Cilic held an edge in points (152-143), aces (19-6) and winners (57-28), but he also made 25 more unforced errors (70-45). In fact, the majority of the statistics favored Cilic in this match—just not the only one that mattered.

The final line.

While Nishikori does not have a Grand Slam title on his resume as of yet, he continues to make deep runs in New York. This is the third time he has reached the semifinals in his last four U.S. Open appearances, making it all the way to the finals back in 2014. He lost to Stan Wawrinka in four sets in his last semifinals appearance.

Meanwhile, Cilic failed to reach his third semis in the last five years. He won the 2014 U.S. Open for his only career major title.

Nishikori will face Novak Djokovic in the semifinals on Friday.

No. 6 Novak Djokovic def. John Millman, 6-3, 6-4, 6-4

Novak Djokovic is one step closer to playing for his third U.S. Open title, although his latest victory didn't come without some weirdness.

The 31-year-old snagged the first set from John Millman, who took down Roger Federer in four sets on Monday:

It was in the second set, though, where there was a rather unusual delay. With the set tied at two games apiece, Millman requested a pause in the action because he was sweating so much that he needed to change his outfit, via SBS Sport:

Millman wasn't the only one to take advantage of the break, though:

While a new outfit may have made Millman more comfortable and been more functional, it couldn't help him topple the two-time champ.

Djokovic came out of the break by winning four of the next six games to take a two-set lead. 

The key? Djokovic won 81 percent of his first-serve points, while Millman won just 65 percent. That helped him stay in control, with the help of his ability to win 70 percent of his second-serve points as well.

In the third set, Djokovic put the finishing touches on the victory:

It marked the second time this year Djokovic eliminated Millman in straight sets. Djokovic won, 6-2, 6-1, at the Queen's Club back in June. Millman may not have been able to steal a set in this match, but he was at least able to make things more competitive this time around.

This victory continues an incredible streak for Djokovic. Excluding when he missed the tournament last year due to an elbow injury, it marks the 11th straight U.S. Open semifinal for the Serbian star.

He rides serious momentum into the semis this year, having swept his last three matches. In fact, he has lost just two sets all tournament.

Djokovic will face Kei Nishikori on Friday for a spot in the finals.