Novak Djokovic eyes a return shot toJuan Martin del Potro during Thursday’s quarterfinal. (Darron Cummings/ASSOCIATED PRESS)

It’s difficult to imagine two players laboring more brilliantly or tirelessly on a tennis court than Juan Martin del Potro and Janko Tipsarevic did at Arthur Ashe Stadium on Thursday at the U.S. Open. But both came out on the losing end of back-to-back men’s quarterfinals that packed a tournament’s worth of dazzling shot-making and drew multiple standing ovations in the process.

Moving on to Saturday’s semifinals are the tournament’s defending champion, Novak Djokovic, 25, who blistered one line after another in defeating del Potro 6-2, 7-6 (7-3), 6-4. The 3-hour, 6-minute match was no ordinary straight-sets affair, with one seemingly unreturnable shot following another.

Djokovic hit 43 winners compared to 28 unforced errors. Against nearly every other player on tour, del Potro’s tally of winners would have been as high. But Djokovic’s defense was outstanding.

With the victory, the Serbian advanced to his 10th consecutive semifinal in a major. He’ll take on Spain’s David Ferrer, 30, who was treated for blisters following his victory over Tipsarevic, which took more than 41 / 2 hours. Ferrer rallied from a 1-4 deficit in the fifth set.

Ferrer’s rally came after Tipsarevic took a nasty tumble, landing hard on his left hip. That was just one of multiple turning points in the most gripping match of the tournament. Ferrer broke his serve, hung in to force a tiebreaker and prevailed 6-3, 6-7 (7-5), 2-6, 6-3, 7-6 (7-4).

“I was trying to play aggressive,” said Tipsarevic, 28, who played the higher stakes game, hitting 60 winners to Ferrer’s 49, but committing more unforced errors (58 to 44). “I needed to play my best tennis staying close to the line. When that was going, I really enjoyed myself.”

The effort by both players was so relentless that the crowd stood in cheered as the decisive tiebreaker got underway. It was remarkably well played, despite coming more than four hours into the match.

One mini-break was all Ferrer needed, and he served out the victory from there, then feel to his knees in disbelief before jogging to the net to share a warm embrace with Tipsarevic.

“My opponent, he deserve also to win this match, no?” said Ferrer, the No. 4 seed. “In one tiebreak, it’s a lottery. And I had luck in important moments.”

Djokovic also praised his opponent after the two embraced at the net when their match ended before an awestruck crowd. Even three-time U.S. Open champion John McEnroe, the lead analyst in ESPN’s broadcast booth, gushed with praise. “These are the days when it’s a pleasure to be a commentator,” McEnroe said, “when you see guys go out and play that quality tennis.”

With the 6-foot-6 del Potro getting off to a typically sluggish start, it wasn’t surprising that Djokovic won the opening set. But del Potro, 23, seized the advantage and served for the second set at 5-4. Djokovic refused to fold, blasting back everything the Argentine sent his way.

It was the third time the two had met in the last two months. Del Potro defeated Djokovic to win the bronze medal at London Games in early August, then fell to the Serb on hard courts of Cincinnati a few weeks later.

Djokovic has yet to lose a set and is two victories from claiming his second major of the year and his sixth overall.

Saturday’s other men’s semifinal will pit No. 3 seed Andy Murray against Tomas Berdych.