When del Potro is healthy, and his shots are on target, he's as dangerous as they come on the sort of hard courts used at the U.S. Open.
Getting his game in shape ahead of the year's last Grand Slam tournament, the top-seeded del Potro solved Isner's big serve and pulled away for a 3-6, 6-1, 6-2 victory in the Citi Open final Sunday, winning his third championship and 14th consecutive match in Washington.
"His forehand is widely regarded as maybe the best in the world," Isner said. "I couldn't do anything about it. I actually wasn't really surprised. I've seen him do that a lot of times. That's what he does. He's got a lot of talent, and a lot of talent on that forehand side."
Back in 2009, del Potro followed up a title in the nation's capital with a trophy at the U.S. Open, which starts in three weeks.
"Fantastic memories," the seventh-ranked del Potro told the crowd.
When the Argentine arrived in Washington this time, he came with some questions about his body and his game. He hadn't played a hard-court match since the spring. He hadn't competed at all in nearly a month -- a five-set loss July 5 to No. 1 Novak Djokovic in the longest semifinal in Wimbledon history, more than 4½ hours. And he hadn't fully tested the left knee he hyperextended in a tumble during a match at the All England Club.
"I'm surprised to win this tournament," del Potro said.
He came through without any problems, other than that first-set hiccup, which might have been due to a short turnaround: His semifinal was delayed by rain Saturday night and ended after midnight. He didn't get to sleep until about 3 a.m., 12 hours before the final started, which he said was "a little frustrating."
In the women's final, seventh-seeded Magdalena Rybarikova of Slovakia beat unseeded Andrea Petkovic of Germany 6-4, 7-6 (2) to win the Citi Open for the second consecutive year. The 43rd-ranked Rybarikova is 10-0 in Washington, where she's won half of her four career titles.
With wind whipping around at 15 mph and temperatures at 80 degrees, Isner got off to a terrific start Sunday -- thanks in part to his powerful and always effective serve, not surprisingly, but also with other aspects of his still-evolving game.
It took Isner less than 10 minutes to create a cushion, breaking at love for a 3-1 edge and soon was up 5-2. Given that Isner never had won a set against del Potro in three previous meetings, it made sense that nothing would come easily this time, either. So Isner required five set points before eventually cashing in with a 137 mph service winner.
And then, everything changed. Perhaps Isner was bound to falter after such impeccable serving all week. Perhaps the fatigue of playing his ninth match in 11 days -- the American won a hard-court title at Atlanta last week -- caught up to him.
After all, Isner thanked his chiropractor during the trophy ceremony, drawing chuckles from fans.
Del Potro's take: "It was strange to hear that."
While Isner acknowledged he was tired, del Potro had a lot to do with the way the match shifted.
Starting off way behind the baseline to receive serves and using every bit of his considerable wingspan, the 6-foot-6 Del Potro broke Isner four times in all, including three in a row while winning seven consecutive games to take control.
Isner -- best known for winning the longest match in tennis history, 70-68 in the fifth set at Wimbledon in 2010 -- had not even faced a break point over any of his previous three matches.
Del Potro also limited Isner to six aces after the eighth-seeded and 20th-ranked American pounded 29 in the semifinals.
"I'm sure that was his game plan, just to get my serve back and go from there," Isner observed. "He was standing so far back that the guy calling the sideline had to move out of the way."
Del Potro added to his Washington titles from 2008 and 2009. He skipped the tournament for various reasons from 2010-12.
He earned his first break point Sunday -- and the first Isner faced since his opening match -- 35 minutes into the final, in the second set's second game. But a 14-stroke point ended with del Potro sailing an easy forehand long for an unforced error. He hung his head, leaned over with hands on knees, then crouched down and stared at the net.
Isner also took the next two points to hold for 1-all. But Isner wouldn't win another game for a substantial stretch.
A bad drop shot into the net, a poor overhead smash right at del Potro that resulted in a cross-court forehand winner, a long forehand and a long backhand all added up to a break at love. That ended a run of 50 consecutive service games won by Isner across four matches and put del Potro up 3-1 in the second set.
Clearly, he was in charge. After only 1½ hours, the final was finished.
Next for del Potro are hard-court tournaments in Montreal and Cincinnati. Those lead up to the U.S. Open, where Isner said del Potro will be "certainly one of the top favorites."
Hard to argue at the moment.