John Isner hits a running forehand during his 6-7 (9-7), 6-3, 6-4 comeback win over Dmitry Tursunov in the semifinals of the Citi Open on Saturday. Isner, who won the Atlanta Open last weekend, also reached the Citi Open final in 2007. (Jonathan Newton/The Washington Post)

It was just past midnight when Juan Martin del Potro earned his spot in Sunday’s Citi Open final, where one of the few players taller than he will be waiting.

The 6-foot-6 del Potro, a two-time champion of Washington’s hard-court event, will take on American John Isner, a 6-9 ace-blasting machine who had a tougher time earlier in the day securing his spot in a second final in as many weeks.

As is his custom, Isner drew on his favorite shot to dig out of trouble after losing a first-set tiebreak to Dmitry Tursunov, firing 29 aces en route to a 6-7 (9-7), 6-3, 6-4 victory.

Saturday evening’s semifinal, pitting top-seeded del Potro against third-seeded Tommy Haas, was halted by rain for 3 hours 10 minutes. Haas held a 4-1 lead in the opening set when the rains came. But when players returned to the court, del Potro was the better player and closed, 7-6 (7-4), 6-3, to reach the final without losing a set through four rounds of play.

Isner, 28, has never beaten del Potro, losing all three of their previous meetings in straight sets. But the two haven’t faced each other in three years.

After slogging through a 52-minute opening set only to lose it in a tiebreak, the eighth-seeded Isner had every reason to slip into a stew of self-recrimination at William H.G. FitzGerald Tennis Center in Rock Creek Park. But he had dug out of the same deficit Friday to earn his spot in the semifinals. So when the scenario repeated itself 24 hours later, with a place in the final at stake, Isner wasted no time sulking.

Isner has won 11 of his past 12 matches, suggesting he could be a factor at the U.S. Open, which gets under way Aug. 26 in New York. He excels on hard courts, but has yet to advance beyond the quarterfinals of the season’s final major.

A native of Greensboro, N.C., Isner reached the final here in 2007, falling to Andy Roddick. Though he has improved his fitness since then and shored up his chief vulnerabilities, his service return and backhand, Isner hadn’t managed to return to the final in Washington — until Saturday.

Having won the hard-court title in Atlanta last Sunday, Isner conceded he’s a bit tired. Between that tournament and this, he has played eight matches in the last 10 days. He has been icing his knees after each match, and midway through the third set Saturday he called for the trainer to massage a quadriceps muscle but later downplayed the significance.

“My body doesn’t feel great, but at the same time nothing is wrong with me besides just being a little tired and worn out,” Isner said. “That’s what happens when you play well. I won last week and have been playing well this week. When you’re winning, you’re playing a lot of matches, and it’s going to wear me down. Me being so big and weighing a lot, I wear down quicker than most guys.”

In many respects, the unseeded Tursunov was the opposite of Isner, compensating for a shaky first serve with an all-around game and solid defensive skills.

Their opening set was a seesaw affair that lacked a single shift of momentum — just players taking turns holding serve until they reached the tiebreak that seemed inevitable from the outset.

Isner fired a 142-mph ace to fend off one set point. But the American played more cautiously when confronted with a second set point. Tursunov, 30, drove him well beyond the right baseline with a huge forehand, then followed with a forehand volley into the open court to claim the first set.

But the Russian found himself under fire from the start of the second set, facing just his second break point of the match. Yet he held serve, helped by an errant forehand by Isner, who roared in frustration.

Still, Isner had the momentum when rain halted play with Tursunov serving at 1-2 and 30-40.

After a delay of more than 70 minutes, the proceedings resumed. And Isner, who had showered, stretched and snacked during the break, pounced on Tursunov.

In the evening semifinal, Haas tried keeping the pressure on the Argentine once play resumed, blasting huge serves and charging the net. But he wasn’t as precise as before, and he slammed his racket in disgust after a double fault at 4-4. He was broken again in that game.

In the first of the two women’s semifinals, Magdalena Rybarikova locked down one spot in the women’s final with a 6-2, 6-0 win over Ekaterina Marakova in a match that was delayed more than three hours by rain.

A smooth return game allowed Rybarikova to control the match. Marakova had reached the semis after playing just one game in the quarterfinal against Monica Niculescu, who withdrew with a wrist injury.

Rybarikova, No. 43 in the WTA rankings, will take on the winner of the Alize Cornet-Andrea Petkovic match, which started well after midnight.

Chelsea Janes contributed to this report.