NEW YORK (AP) - Locked in a taut, thrill-a-minute second set, Novak Djokovic and Juan Martin del Potro headed to a tiebreaker and produced a 20-stroke masterpiece of a point befitting a pair of past U.S. Open champions.
More than a dozen shots in, defending champion Djokovic drew del Potro forward with a drop shot, then tossed up a lob. Del Potro, the 2009 champion, sprinted with his back to the court, got to the ball and lofted a lob the other way. Djokovic slammed an overhead. Del Potro somehow kept the ball in play. Djokovic laced a drop shot. Again, del Potro got there, attempting another lob. It landed long.
A point from a two-set lead, Djokovic threw his head back, roared "Come on!" and pumped his arms. Del Potro leaned his elbows atop the net, hunched over and rested his head on his arms.
Close and compelling as their quarterfinal was, it might as well have been over right then and there. Djokovic's down-the-line backhand winner seconds later ended the tiebreaker and gave him a commanding lead on the way to a 6-2, 7-6 (3), 6-4 victory over del Potro on Thursday night that put the Serb in his 10th consecutive Grand Slam semifinal.
"We played some incredible rallies and incredible points," the second-seeded Djokovic said. "It's always entertaining, always so much fun, playing in these night sessions."
Djokovic will face fourth-seeded David Ferrer on Saturday, with a spot in Sunday's final at stake. Ferrer advanced to his fourth career major semifinal by using his own version of leg-churning, ball-chasing tennis to outlast eighth-seeded Janko Tipsarevic of Serbia 6-3, 6-7 (5), 2-6, 6-3, 7-6 (4) in 4 hours, 31 minutes.
Olympic champion Andy Murray and 2010 Wimbledon runner-up Tomas Berdych - who eliminated 17-time major champion Roger Federer - earned their semifinal berths Wednesday.
Ferrer needed a mid-match pedicure of sorts, seemed bothered when Tipsarevic got a mid-game medical timeout and, worst of all, was down 4-1 in the fifth set.
In the end, though, the indefatigable Spaniard was barely better, as he usually is when matches go the distance. He has won four consecutive five-setters and is 17-9 overall.
"I don't have words," said Ferrer, who reached the semifinals at the French Open in June. "It was a very emotional match."
When it ended on Tipsarevic's backhand into the net, Ferrer raised his arms, then knelt near the baseline. The weary foes met at the net for a hug.
"David is a fighter. He is one of the biggest competitors we have in the game. People ... overlook him," Djokovic said. "You need to earn your points against him."
Hmmmm. Sound familiar?
Del Potro might use those exact words to describe Djokovic.
Under the lights at night in Arthur Ashe Stadium, the showcase matchup of Djokovic's squeaky-sneaker defense, reflex returns and line-catching groundstrokes against the seventh-seeded del Potro's big-as-can-be forehands topping 100 mph lasted a little more than three hours. It was tremendously good every step of the way, and the second set alone was 84 minutes long - 11 minutes more than Djokovic's entire first-round match last week.
"Crucial," Djokovic said about Thursday's second set. "It could have gone either way."
Djokovic and del Potro, a 6-foot-6 Argentine, each was left smacking himself in the head with a racket after an ugly mistake - the shot-making was so high-caliber that any miscue really stood out. The full house responded more than once with standing ovations, but the loudest and longest came before the tiebreaker.
These are the only two men who have managed to beat Federer and Rafael Nadal, owner of 11 major trophies, in the course of a single Grand Slam tournament. Djokovic and del Potro each did it while on the way to the championship at Flushing Meadows.
In the three years since del Potro's triumph, though, he hasn't reached another major final. He was sidelined for most of the 2010 season - and missed out on a chance to defend his U.S. Open title - because of a right wrist injury that required surgery. He's had problems with his left wrist recently, limiting the effectiveness of his two-fisted backhand.
Still, he beat Djokovic for the bronze medal at the London Olympics last month and gave the guy all he could handle throughout the second set.
"I didn't expect to win easy, that's for sure," Djokovic said.
In what would turn out to be the match's final game, del Potro - who ended Andy Roddick's career by beating him Wednesday - at least got a chance to briefly celebrate on what has become Djokovic's territory, smacking a brilliant backhand winner and hopping on a courtside advertising wall while raising his arms overhead.
Fans leaped out of their seats. Even Djokovic had to acknowledge the shot, using his racket to applaud.
But otherwise, this was Djokovic's night to shine. He is into the U.S. Open semifinals for the sixth year in a row, and the streak of 10 he has at all major tournaments - dating to Wimbledon in 2010 - ties Rod Laver and Ivan Lendl for the second-longest, behind Federer's record of 23.
Djokovic is bidding to raise his Grand Slam trophy collection to six, and on Thursday he displayed all of the skills that have carried him to the top. He dulled del Potro's intimidating serves, accumulating 13 break points and converting four. He served well enough to face only three break points himself, saving all but one. And he grabbed control of lengthy baseline back-and-forths, racing and stretching and contorting his body to get to shots del Potro figured would have been winners against just about anyone else.
Del Potro said Nadal is the only man whose ability to track down tough shots and get them back over the net to extend a point can be compared to Djokovic's.
"Not many players can do that. ... It's amazing," del Potro said.
A few times, perhaps more hoping than truly believing a Djokovic shot was out, del Potro challenged calls, only to have the replay confirm the ball was in. After one, del Potro stood with his hands on his hips, shaking his head while staring up at the overhead videoboard.
"I wanted to destroy that machine," del Potro said. "But what can you do?"
In that marvelous second set, del Potro led 5-3, and served for it at 5-4 but got broken. Then came a game of more than 15 minutes that included a bunch of riveting points, when del Potro served while suddenly trailing 6-5. He needed to save three set points and did, sending it to the tiebreaker.
Djokovic emerged there. When the set ended, del Potro grabbed a black gym bag and headed to the locker to change his sweat-soaked clothes.
"I had the chance to win that set," del Potro said, "but Nole played really well, basically, in the important moments."
The only remaining question about the outcome arose early in the third set, when Djokovic winced and clutched at his left side after lunging for a backhand. He flexed his back a bit, and stomped his feet, and played well enough to avoid fretting about any sort of comeback.
"He's playing better and better," del Potro said, "every day and tournament."
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