Serena Williams into semifinals

— -- NEW YORK -- Off to a bad-as-can-be start to her US Open quarterfinal, losing serve twice while dropping the first three games, even Serena Williams found it hard to believe.

"I was thinking: 'I'm down two breaks?" she said. "But I ... felt like, 'It's not the end of the world."

Quickly turning things around and taking complete control, Williams stretched her Flushing Meadows winning streak to 19 matches by eventually overpowering 11th-seeded Flavia Pennetta of Italy 6-3, 6-2 on Wednesday night.

"I don't feel like I was doing too much wrong," said Williams, a five-time champion at the US Open, including the past two years. "So I said, 'If she keeps it up, she absolutely deserves the win.' And I just tried to do a little better."

Simple as that, huh?

"Against her, you can't wait, you can't give her time, you can't fail to push her to her limits -- because if you do, she's a runaway train," Pennetta said.

Williams is bidding to become the first woman with three consecutive US Open titles since Chris Evert won four in a row from 1975-78. The 32-year-old American also is trying to pull even with Evert and Martina Navratilova at 18 Grand Slam singles trophies.

Williams had not yet reached a major semifinal in 2014, bowing out in the fourth round at the Australian Open, the second at the French Open, and the third at Wimbledon. The last time she didn't reach at least one Grand Slam title match in a season was 2006, when she entered only two of the sport's top tournaments.

"Well, honestly, I've had a tough year in the majors and I've (lost to) some great players that weren't in the top 10," Williams said. "So you can never underestimate anyone."

On Friday, Williams will meet 17th-seeded Ekaterina Makarova, who beat 2012 and 2013 runner-up Victoria Azarenka 6-4, 6-2. The other women's semifinal will be No. 10 Caroline Wozniacki of Denmark against unseeded Peng Shuai of China.

One of these is not like the others: Williams is the only member of that quartet with a Grand Slam title. Wozniacki, the runner-up at the 2009 US Open, is the only other woman left who's even participated in a major semifinal.

On her second serve, she was called for a foot fault -- an unpleasant reminder of her meltdown after that very same ruling in the closing moments of a loss to Kim Clijsters in the 2009 US Open semifinals.

This time, Williams was unfazed right afterward and wound up winning the point. But miscues by Williams led to an opening break, and after about 10 minutes of play, Pennetta -- a semifinalist in New York last year, but never a major finalist -- was ahead 3-0.

Williams began taking the ball inside the baseline as much as possible and finding the mark with her serves, putting more pressure on Pennetta while reeling off six straight games to take the first set.

Pennetta, who is into the doubles semifinals with partner Martina Hingis, might have been forgiven for giving up at that point. But she made things competitive again -- at least briefly.

Four aces in one game allowed her to lead 2-1 in the second set. That was pretty much that. Williams broke at love to go up 3-2, raising her left fist overhead to celebrate one particularly impressive shot, in which she raced back to the baseline to retrieve a lob, spun and smacked a forehand winner.

Williams has not had a particularly difficult path so far through an upset-filled women's field.

She hasn't dropped a set, but she also has not had to face No. 3 Petra Kvitova, No. 7 Eugenie Bouchard, No. 8 Ana Ivanovic, No. 16 Azarenka or No. 24  Samantha Stosur -- the last player to beat Williams at the US Open, in the 2011 final.

All were on Williams' half of the draw, and all lost to others.

Information from The Associated Press was used in this report.