Second-ranked Simona Halep has the longest current run in the world top 10, reached the second grand slam final of her career in June and defends a title next week in Toronto.
But the 25-year-old Romanian is looking for the big prize that has eluded her, a major title, as she begins working to the US Open at this week's WTA and ATP Citi Open.
Halep is getting an early jump on US Open preparations at the Washington hard-court event, becoming the highest-ranked player ever in the field after she said this week's WTA event in Stanford denied her a wildcard entry.
"I want to be in my best form at the US Open," Halep said. "This tournament is very important. Every match is tough and important for my level.
"I'm focused on what I have to do on the court and what I need to do to get my game strong."
Halep reached her first grand slam final in 2014 at Roland Garros, falling to Russia's Maria Sharapova, then made it back to the French Open final this year only to fall again, up a set and 3-0 before squandering the lead and falling to Latvia's Jelena Ostapenko.
She followed with a quarter-final exit at Wimbledon and after a title defence next week in Canada hopes to improve upon her best US Open result, a 2015 semi-final run.
"I'm still confident," Halep said. "But now it's a different season. I expect some tough matches. I'm ready. I want some matches."
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Halep's first match will be on Tuesday against another competition-hungry wild card, American Sloane Stephens, who missed 11 months after left foot surgery before returning at Wimbledon. Stephens won her first WTA title at Washington in 2015.
On the men's side, five of the top 11 players in the world will take part, including four past champions who won titles in their last Washington appearances - Argentina's Juan Martin del Potro, Canada's 10th-ranked Milos Raonic, Japan's ninth-ranked Kei Nishikori and defending champion Gael Monfils of France.
"The high level of players make this a very important tournament," Del Potro said. "It's going to be a very tough tournament."
Austrian Dominic Thiem, ranked seventh, is the men's top seed with 20-year-old German Alexander Zverev seeded second and moving on Monday into a career-best world No 8 level.
"Of course it's nice to be top seed here but it doesn't affect anything because the guys behind me are really strong. It's a really strong field," Thiem said. "It's going to be tricky first rounds for everybody."
US ninth seed John Isner, a three-time Washington runner-up, withdrew on Monday with a sore right knee. He won ATP titles the past two weeks at Newport and Atlanta.
South African-born Australian Marc Polmans was the qualifying lucky loser who replaced Isner in the draw with a first-round bye.
Isner jumped past Jack Sock to once again become the US No 1, reaching 18th in the rankings to 19th for Sock, who could have faced Isner in the third round.
