Trying to predict women's tennis? Don't, because parity reigns

Keys cruises past Suarez Navarro to US Open semifinals (0:31)

Madison Keys and Carla Suarez Navarro get into a rally and Suarez Navarro's return goes long as Keys wins the match in straight sets 6-4, 6-3. (0:31)

NEW YORK -- After losing in the second round at Wimbledon earlier this summer, Eugenie Bouchard was bombarded with questions about her low ranking (then No. 188) and her inability to consistently win despite her previous success.

She didn't hesitate in her response.

"The top 100, even 200, the difference in level is incredibly small," she said. "Just look at all the upsets we've seen over the top 10 players. The difference between players is more mental than physical at this point. It's almost like anyone can beat anyone on any given day."

Her words were true at Wimbledon, where none of the top five made the round of 16 for the first time in tournament history. It couldn't feel any truer now nearing the end of the US Open.

Entering the semifinal round in Queens, there are no top-10 seeds remaining. Ten of the top 13 were out before the fourth round. Madison Keys, the No. 14 seed, is the highest-seeded player remaining. World No. 1 Simona Halep, the reigning French Open champion, was eliminated in the first round. The other top seeds have been crumbling since -- although that is somewhat of a technicality since Serena Williams remains in the draw. While she is the No. 17 seed, she hardly feels like it with 23 Grand Slam titles to her name.

Garbine Muguruza, the 2016 French Open and 2017 Wimbledon champion, was the third-ranked player in the world in early July. But that meant nothing this summer. She lost in the second round at both Wimbledon and the US Open -- to players ranked No. 47 and 202, respectively.

At the All England Club, Karolina Pliskova was the only top-10 player remaining entering the second week of the tournament. She lost in the fourth round. The 26-year-old was the last remaining top-10 seed in New York, as well, before losing in the quarterfinals to Serena Williams. She wasn't exactly thrilled by the strange distinction when asked about it after her loss on Tuesday.

"I'm not proud -- I would love to go [further]. I always I want to be the one who is in the draw," she said. "There are a lot of different players which were top 10 before and they are not top 10 now. The draw is always very strong. I don't think there's a weak player left, so I'm just happy that I made the second week here."

Pliskova isn't wrong about the strong draw: There have been seven different Grand Slam champions over the course of the past seven Grand Slams. As three of the four US Open semifinalists (Anastasija Sevastova, Naomi Osaka and Carla Suárez Navarro/Keys) have never won a major title, there is a real chance to see eight different champions in the two-year span for the first time in Open era history.

Upsets have become a regular occurrence in the women's game, seemingly much more so than among the top players on the men's side. Roger Federer's fourth-round loss at the hands of John Millman this week was surprising since it was his earliest exit at a Grand Slam since the French Open in 2015. But consider this: There isn't a single woman ranked in the top 20 who hasn't been sent packing before the fourth round during at least one major this year.

As the WTA potentially looks to crown its eighth champion in as many majors, there have been eight different Grand Slam winners in the ATP in the past 13 years. Yup, you would have to go back to Marat Safin's title at the 2005 Australian Open to find an eighth name. The Big Three (Federer, Rafael Nadal and Novak Djokovic) have won 46 of a potential 55 trophies. And if you include Andy Murray and Stan Wawrinka, five players have won all but three titles in that span. There have been 21 different champions on the women's side in that time.

With the (large) exception of Serena Williams' well-documented dominance, it's been anyone's game on the tour for the past several years. That was never more evident than during her yearlong absence for pregnancy and childbirth. Jelena Ostapenko, ranked No. 47, and Sloane Stephens, ranked No. 83, were unseeded during their respective 2017 title runs at the French Open and US Open. The last unseeded men's champion? Gaston Gaudio at the 2004 French Open.

Former World No. 2 and Wimbledon finalist Agnieszka Radwanska, who lost in the first round in Queens, discussed the increased depth on tour last season in an interview with FiveThirtyEight.

"A couple of years ago, those first two rounds when you didn't play against seeded players, it was easy," she said. "You don't have to play 100 percent and you're gonna win. Obviously it's not gonna happen anymore in tennis right now. You can play those players that you really don't want to play in the first round, and that's why I think we can also see a lot of upsets in early rounds."

So why does such parity exist in the women's game. Is it the increase in money at tournaments and from endorsements that allows more players to have access to the best resources and people around them? Or, is it the longevity of careers, thanks in large part to advancements in sports science, that has helped more players be able to win at any given time? Or, perhaps, is it because the best-of-three-sets format allows a better chance for a player to pull off an upset? It's impossible to say definitively.

Eighteen-time Grand Slam champion Martina Navratilova raised a few eyebrows with her controversial theory about the prevalence of upsets in an interview with Sports Illustrated during the Wimbledon chaos.

"Too many things are done for these players today," she said. "They don't do anything for themselves except for hitting the ball. The coaches, the drivers, the physios. They motion, and someone brings them a towel. I saw a player hold out her arm and someone applied sunblock. They don't have to take responsibility.

"Then they get out there [in a Slam] and it's only them. And when the you-know-what hits the fan, and they have to make all these decisions -- what shot to hit, how to adjust -- they are not prepared."

Stephens, the reigning US Open champion, didn't offer much, but perhaps her response after her quarterfinal loss summed it up best.

"There are a lot of great players left in the tournament, so who knows what's going to happen," she said. "It's tennis."