The six-time US champion did not start well as she looked a bit out of sorts with too many errors by her high standards. Meanwhile her Bulgarian opponent played a very consistent game and broke Williams in the fifth game to take the first set.However, the 23-time grand slam champion brought her two decade long experience to good use and counter-attacked her unranked opponent in the next two sets to seal the encounter and make it to the last four at the US Open for the fourteenth time in her career.
Williams will now take on one of her fiercest rivals and former world number one Victoria Azarenka, whom she defeated twice in the finals of the US Open in the 2012 and 2013. Azarenka trounced sixteen seed E Mertens 6-1,6-0 to book her spot in the semifinals.
After her win over Pironkova, Williams said she could not afford another sluggish start if she is to reach her fifth Grand Slam final in three years.
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"I was feeling it a little in my legs and an hour in, for some reason, I got more energy. I can't do that if I want to keep winning," said the American, who last won the US Open in 2014.
"I play again on Thursday. I am used to playing back to back to back, but I need to figure out how to start a little bit faster. "I keep fighting, I never give up. You've got to keep going."