NEW YORK (AP) -- There are so many ways to measure how Serena Williams stands apart from the three other women who will be playing in the U.S. Open semifinals Friday.
Williams already has participated in 24 previous major semifinals. The others' total is four - all by Wozniacki.
Williams has 62 titles from all tournaments, five in 2014. The others have a combined 24, two this year.
On and on it goes.
Still, the top-ranked Williams is all too aware that she has not been at her best during Grand Slam tournaments this season, losing in the fourth round at the Australian Open, the second at the French Open, and the third at Wimbledon. Not since 2006 has she failed to reach at least one major final in a season.
''For me, it's just about: Every day is a new day. You never know. Especially for me this year, you never know what can happen,'' the 32-year-old American said. ''I take everything very seriously as it comes.''
Trying to become the first woman since the 1970s to win three consecutive U.S. Open titles, Williams takes a 19-match winning streak at Flushing Meadows into Friday's semifinal against the 17th-seeded Makarova.
Before that, the 10th-seeded Wozniacki meets the unseeded Peng.
Here's what to watch on Day 12 at the year's last Grand Slam tennis tournament:
WILLIAMS VS. MAKAROVA: It's not surprising that Williams has made it to the semifinals without losing a set. But neither has Makarova, a 26-year-old Russian who never before has been this far in 29 Grand Slam appearances.
And Makarova had by far the tougher path to the semifinals, eliminating Wimbledon finalist Eugenie Bouchard and Victoria Azarenka - a two-time Australian Open champion and the runner-up to Williams at the U.S. Open in 2012 and 2013.
Because of a string of surprises in earlier rounds, Williams was able to avoid facing Bouchard or Azarenka, of course, but also past Grand Slam champions Petra Kvitova, Ana Ivanovic and Sam Stosur.
Williams and Makarova have played each other four times previously, always on hard courts. While Williams has won three of those - including at the U.S. Open in 2012, and at Dubai in February - Makarova did win in the fourth round of the 2012 Australian Open.
Not surprisingly, Makarova remembers that victory well.
The key to beating Williams?
''Definitely to stay aggressive,'' Makarova said. ''Because she's so aggressive, yes? And I need to be a little bit more aggressive than she is.''
WOZNIACKI VS. PENG: Denmark's Wozniacki is 5-1 against China's Peng, whose only head-to-head victory came way back in 2007, when Wozniacki was all of 17.
Two years later, Wozniacki made the U.S. Open final on her way to reaching No. 1 in the rankings. But she hasn't returned to those heights since.
''It was such a huge experience for me and it was my first really deep run in a Grand Slam,'' Wozniacki said. ''I'm just so happy to be back (at) this stage.''
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