John Isner made it to the final at last year’s Citi Open, but fell to Juan Martin del Potro. (Dani Pozo/AFP/Getty Images)

Top-ranked American John Isner and the world No. 1 doubles team of Bob and Mike Bryan are among the early commitments for Washington’s Citi Open, the annual hard-court classic at Rock Creek Park’s William H.G. FitzGerald Tennis Center.

They’ll be joined by former world No. 1 Lleyton Hewitt and France’s Richard Gasquet, with the rest of the field announced by mid-June.

Formerly known as the Legg Mason Classic, the event has expanded to include a women’s tournament that draws rising stars such as 16th-ranked Sloane Stephens, who plans to return this summer.

The 6-foot-10 Isner, 29, enjoyed his breakthrough as a pro at the 2007 Legg Mason just months after graduating from Georgia, where he was an NCAA standout. As a wild card in the event, he won five successive matches on third-set tiebreaks to reach his first final of an ATP event, falling to Andy Roddick in straight sets.

Isner, currently ranked No. 11 in the world, returned to the Citi Open final last year, only to fall to Argentina’s Juan Martin del Potro in three sets. It was del Potro’s third title in Washington.

The 36-year-old Bryan twins have rewritten doubles history, collaborating on 15 Grand Slam titles (three Wimbledons, six Australian, two French and four U.S. opens) as well as the 2012 Olympic gold.

Hewitt, the 2004 Legg Mason champion, remains a tenacious fighter at 33.

Qualifying matches will be contested July 26-27, with the main-draw matches beginning July 28. The men’s and women’s finals are Aug. 3.

With a $1.4 million purse, the Citi Open is one of 11 ATP 500-level events, which places it among the 20 most prestigious tournaments based on prize money and caliber of the field. Coming three weeks before the season’s final Grand Slam, it’s also regarded as a key tuneup for the U.S. Open, to be held in New York Aug. 25-Sept. 7.

The women’s tournament is part of the WTA International Series, which is a rung below the elite tour, and has a purse of $250,000.