American Madison Keys — ranked 27th in the world and seeded seventh at the Citi Open — suffered a first-round upset to Kurumi Nara on Monday. (Steve Bardens/Getty Images)

If you crunch the numbers for American women’s tennis over the past five years, you’ll inevitably come up with “one” and “nine.” One female American tennis player has won a Grand Slam event since 2008, and she has won nine of them: Serena Williams sums up the success of American women’s tennis players at the highest levels for more than half a decade.

But a look at the Citi Open main draw shows a shifting equation: Of the 32 players in the field, nine were Americans, all of them under 25. The trend’s not limited to Rock Creek Park. A growing corps of young Americans are emerging as legitimate WTA threats, and carrying with them hopes for a deeper, more balanced, and more dominant American tennis future.

“We probably can’t impact where the next Serena is coming from,” USTA general manager of player development Pat McEnroe said. “It’s our goal not to rely on that.”

Eleven Americans are ranked in the latest WTA top 100, seven of them younger than 25. Just three other countries have seven players of any age ranked among the top 100. Thirteen Americans are ranked in the top 100 for juniors, including two of the world’s top five. No other country has as many.

“I think [the Americans] are in a great place,” said Juan Todero, who coaches 19-year-old Madison Keys (No. 27 in the world) and 20-year-old Lauren Davis (No. 43). “There’s a lot of young Americans coming up from number 200 to 100 and 100 to number 1. They’re getting closer.”

Guide to the 2014 CitiOpen Tennis Tournament (The Washington Post/The Washington Post)

Keys and 21-year-old Sloane Stephens (No. 22), the two players nearest the WTA elite, headlined this year’s Citi Open field. Stephens, an ultra-talented but enigmatic starlet, reached the 2013 Australian Open semifinals, upsetting Williams on the way. Inconsistency stalled her once rapid ascent this season, and Tuesday night she suffered a first-round upset to fellow young American Christina McHale.

Keys, who also dropped a disappointing first set match, hers in straight-sets to Kurumi Nara, won her first WTA tournament this season in Birmingham, England .

But while the two most prominent young Americans stumbled out of the Citi Open early, their exits reveal depth: six of the nine Americans in the Citi Open draw have reached their career-high rankings this season, and three remained active heading into Wednesday’s second round.

“Now they have to work and it’s a learning experience,” Todero said. “. . . I even tell them, some say you have time because you’re young. But then by the time you realize, you’re 26 sometimes. She’s young, but that doesn’t mean she has time. You have to develop and work.”

This year, many young Americans are looking impatient.

Eighteen-year-old Taylor Townsend is validating her fan-favorite status with a strong season including a surprising third-round showing at the French Open. Twenty-two-year-old Shelby Rogers knocked off third-seeded Alize Cornet on Monday night, her third win in her past four tries against top 25 players.

Davis and 23-year-old Alison Riske, who was bounced from the Citi Open in the first round Monday, both reached career highs in the top 100 this season.

McHale, Stephens’s first-round conqueror, took the William H.G. FitzGerald Tennis Center court as the world’s 50th-ranked player, but has risen as high as 24 in recent years. Tornado Alicia Black, No. 4 in the world junior rankings, won her first WTA match Saturday. The 16-year-old then qualified for this week’s main draw with another win Sunday.

The list extends beyond the Citi Open field, to up-and-comers such as 18-year-old Victoria Duval, who broke into the top 100 this season before receiving a diagnosis of Hodgkin’s lymphoma in July. Duval said she expects to make a full recovery. CiCi Bellis, 15, is the world’s second-ranked junior player.

“We’re all awesome,” Rogers said of the young American contingent. “I think that’s why we’re all doing so well. We’re pushing each other, but at the same time helping each other. It’s all positive and a great atmosphere to be around.”

Todero trains players at the USTA facility in Boca Raton, Fla., and sees many of the rising Americans regularly. He said they’re competitive, not all friends, but constantly working together.

“Like coworkers, you could say,” Todero said. “They’re pushing each other. A couple are doing very well, and I think that pushes the rest to do better.”