Novak Djokovic returns a shot during his quarterfinal victory over Juan… (Clive Brunskill / Getty…)
NEW YORK — There was a game that lasted nearly 17 minutes and a moment when Juan Martin del Potro stopped playing to gently place a ground-bound moth on his tennis racket and sweetly send it out of danger.
When Del Potro's massive forehand would hit the surface, it could have smashed much larger living beings. What it couldn't do was slow down Novak Djokovic.
Djokovic, the defending U.S. Open champion and the tournament's second-seeded player, beat Del Potro, the 2009 champion, 6-2, 7-6 (3), 6-4, Thursday night.
Although the score seems routine, the second set lasted 1 hour 24 minutes and was filled with lobs sliding off lines, slippery volleys that made noise as they cut through the humid air, with Del Potro's massive forehands that were so often met by Djokovic's relentless defense.
There were times when Djokovic slipped in puddles of his own sweat, but he might be the fresher player in the semifinals Saturday, where he will find fourth-seeded David Ferrer. Ferrer needed 4 hours 31 minutes to win his quarterfinal against eighth-seeded Janko Tipsarevic, 6-3, 6-7 (5), 2-6, 6-3, 7-6 (4) Thursday afternoon.
"It was a straight-set win, but it was much, much closer and tougher than the score indicated," Djokovic said. "Whoever saw the match would understand. It could have gone really either way. It was a lot of fun to be a part of this great match."
Friday will be devoted to the women's semifinals and even though Serena Williams will be the only American playing, she might not be the fan favorite.
Even after her shouty aces and wickedly determined ground strokes, Williams seems to get murmurs more than ovations here.
It is as if the fans suddenly remember: Wasn't that Serena who made threats against a chair umpire during her women's final loss a year ago to Sam Stosur? Wasn't it an angry Williams who thrust her racket into the face of a lineswoman who had dared to call a foot fault on Williams in her semifinal match against Kim Clijsters in 2009?
After top-seeded Victoria Azarenka of Belarus faces third-seeded Russian Maria Sharapova in Friday's first women's semifinal, Williams will take on 10th-seeded Sara Errani.
Errani, a 5-foot-4 1/2 Italian, is having the best year of her career. She reached the French Open final and had never been past the third round of the U.S. Open until now.
But Williams is trying to make friends and still earn wins.
After she had easily beaten Ana Ivanovic on Wednesday night in the quarterfinals, 6-1, 6-3, Williams, seeded fourth, mentioned a brief loss of focus.
She was asked, "Does it feel good to be at a stage of your career where you can win a quarterfinal of a Grand Slam in less than an hour and chide yourself for not having enough focus?"
Williams' eyes grew big.
"Gosh, that does sound pretty bad, huh? I didn't mean to sound that bad. I was too honest. I think Ana played better in the second set and was definitely putting a lot more pressure on me," Williams said.
It was as if Williams had suddenly envisioned big tabloid headlines about her lacking respect for an opponent.
In fact it is Errani who is receiving extra scrutiny.
Errani has had to answer questions about her association with Spanish doctor Luis Garcia del Moral, who is one of the men recently banned from cycling by the United States Anti-Doping Agency (USADA) for being associated with Lance Armstrong and the U.S. Postal cycling teams that have been investigated for doping.
"Of course, I'm not interested to keep working with one person that is involved in these things," she said. "He was the best doctor in Valencia for everything. But now his name is not a good name."
Williams seems aimed at making hers a good name at this place again.