Serena Williams, of the United States, reacts during a match against Tsvetana Pironkova, of Bulgaria, during the quarterfinals of the US Open tennis championships, Wednesday, Sept. 9, 2020, in New York.
Williams last lost before the semifinals in New York in 2007, when Justine Henin eliminated her in the quarterfinals.
Another 15 strokes followed, with Williams smacking a forehand passing shot that Pironkova volleyed into the net tape.
A couple of times, Pironkova drew Williams forward with a drop shot, then took the point with a perfectly arced lob.
But after compiling only five unforced errors in the first set, Pironkova had 21 over the last two.