The top-seeded Williams took just over an hour to handle Flavia Pennetta of Italy, seeded 12th, 6-3, 6-2. Pennetta won the first three games of the match, then got blitzed, 12-2, the rest of the way.
Williams is seeking her third straight U.S. Open title and her sixth overall. A victory here would redeem a below-par season in the Grand Slams for Williams, who had yet to get to a major quarterfinal before she got to New York.
FOR THE RECORD:
U.S. Open tennis: An article in the Sept. 4 Sports section about the quarterfinals of the U.S. Open said tennis player Victoria Azarenka was from Bulgaria. She is from Belarus.
"I've been working hard all year," Williams said afterward. "In the losses I had, I just kind of blew up."
Williams' opponent in her semifinal will be Ekaterina Makarova of Russia. Her 6-4, 6-2 victory over Victoria Azarenka of Bulgaria was not a huge upset on the draw sheet. She was seeded one spot below Azarenka at No. 17.
But Azarenka had been to the final the last two years here and had won two Australian Opens, so expectations for her may have been a bit higher. Makarova, who has been in three Grand Slam quarterfinals — including here last year — dominated the return games, getting 12 break points and converting four of them.
Azarenka admitted she had suffered from food poisoning the day before, but didn't want that as an excuse.
"I just want to give credit to my opponent," she said. "She played really well. Am I disappointed? Yes, I'm disappointed, but I feel like I tried my best."
The other women's semifinal was established Tuesday and will pit No. 11 Caroline Wozniacki of Denmark against unseeded Peng Shuai of China.
Though the marquee matchup of the day was in the men's singles between Novak Djokovic and Andy Murray in the late night match, Stan Wawrinka and Kei Nishikori put on a great lead-in show in the late afternoon.
They battled for 4 hours 15 minutes, before Nishikori of Japan watched Wawrinka's final shot settle into the net in his 3-6, 7-5, 7-6 (7), 6-7 (5), 6-4 marathon victory over this year's Australian Open champion.
That was even more exceptional because Nishikori had played 4 hours 19 minutes Monday night, finishing at 2:26 a.m Tuesday. That was also a fight-to-the-death five-setter, over Milos Raonic of Canada.
McEnroe stepping down
The director of the U.S. Tennis Assn. Player Development Program, Patrick McEnroe, announced that he would be stepping down from that post, as soon as the new USTA center in Lake Nona, Fla., is completed. McEnroe has held the job for the last six years and was the U.S. Davis Cup captain for 10 years, ending in 2010.