John Isner didn’t face a match point in a three-set victory Friday over Marcos Baghdatis. (Jonathan Newton/The Washington Post)

Having already toppled the tournament’s No. 2 seed, Marcos Baghdatis strode on court at Washington’s Citi Open on Friday feeling good about his chances against John Isner despite the fact the American towered nine inches taller, was ranked 24 spots higher and owned a 4-0 record against him.

The first point disabused him of that notion. Isner blasted the first of 18 aces en route to a 6-7 (5-7), 6-4, 6-4 victory that propelled him into Saturday’s semifinals in Rock Creek Park.

“It’s a completely different sport!” Baghdatis said of the challenge of playing Isner, whose pulverizing serve and forehand snuff out most rallies before they get started — especially on hard courts. “You basically have no rhythm. You play a rally maybe every five minutes, so you lose your rhythm.”

Baghdatis did well to withstand Isner’s barrage, forcing a first-set tie break and claiming the opening set. But his patience unraveled under the relentless pounding that robbed him of any tempo.

“All of a sudden I have no rhythm and start panicking and start giving some points away or pushing the ball, not being aggressive,” said Baghdatis, 28, the Citi Open’s 2010 runner-up. “It’s always the same story against him.”

It’s tempting to dismiss Isner, the top-ranked American, as a player with a single weapon. And it would be easy to dismiss that weapon, his serve, as an accident of birth or little more than a consequence of his 6-foot-9 height.

Both would be wrong, said former touring pro Justin Gimelstob, a Tennis Channel analyst who believes Isner would brandish one of the game’s best serves even if he stood 6-3.

When he looks at Isner’s service motion, Gimelstob sees biomechanical perfection.

“One of the reasons the serve is so tough is because it has so many moving parts,” Gimelstob said. “John has perfect coordination between his lower and upper body. He uses all of the major muscle groups—the back, the shoulder, the legs. He has great drive from his lower body, and he has excellent extension.”

That single stroke is part of what has made Isner among the game’s hottest hard-court players.

But he has called on more than a serve to win 10 of his past 11 matches. With neither the natural grace of Roger Federer nor the dazzling speed of Novak Djokovic, Isner has worked to improve his agility. His service return isn’t the liability it was early in his career. And he’s doing a better job leveraging his uncommon reach, finding occasions to come to the net rather than simply launch one forehand bomb after another from the baseline.

Isner should be helped by the fact that Washington’s hard courts seem faster than in previous years, according to several players. The faster the court, the less time opponents have to react to the ball, which translates to an edge for big servers.

“I don’t really care how fast the court is,” Isner said, downplaying the issue. “I just go out there and try to hold serve.”

All of this should make Isner a handful for Russia’s Dmitry Tursunov, 30, who overcame 13 double-faults earlier in the day to oust Marinko Matosevic, 6-3, 4-6, 7-6 (7-4), in a match that lasted 2 hours 44 minutes.

Friday was a busy day at William H.G. FitzGerald Tennis Complex, where all four men’s quarterfinals and all four women’s quarterfinals were contested.

Third-seeded Tommy Haas advanced with a 7-6 (7-5), 7-6 (7-3) victory over Grigor Dimitrov of Bulgaria. He’ll face top-seeded Juan Martin del Potro, who defeated South Africa’s Kevin Anderson 7-6 (7-0), 6-3, to clinch a spot in Saturday’s other semifinal.

Saturday’s women’s semifinals will pit No. 3 seed Ekaterina Makarova against Magdalena Rybarikova of Slovakia and No. 4 seed Alize Cornet of France against Germany’s Andrea Petkovic. Rybarikova ousted the top seed, Angelique Kerber, 7-6 (7-0). 3-6, 6-3.

Tursunov had great reason to cheer his victory Friday, having reached the semifinals of a tournament just once the year. Making the final four in Washington brought a welcome boost to Tursunov’s confidence and financial bottom line.

Tursunov was born in Moscow but moved to the United States at age 12. He reached a career-high No. 20 in the world in 2006, but his ranking plunged after he underwent three ankle surgeries, hovering near 200.

He has worked hard to climb back up in the rankings. But for an aging player who increasingly relies on the services of a physical trainer, the earnings of the world’s 61st-ranked player barely translate to a break-even proposition once he pays for coaching fees and airline tickets and hotel rooms for himself, a coach and trainer to circle the globe all season.

“If I win this tournament, I can cover my bills for the rest of the year,” Tursunov said. “But I haven’t been winning many tournaments. Sometimes players split coaches or split physios. It’s like budgeting a family budget. It’s not like every week we show up and get $30,000 in our pocket.”