WIMBLEDON, England — The quarterfinal everyone expected, a dream battle pitting newly minted women’s No. 1 Ashleigh Barty against Serena Williams, seemed oh-so-very-close as play began at the All England Club on Monday morning.

Williams seemed a safe bet to do her part. She was facing Carla Suárez Navarro, to whom she had never lost a set in six previous matches.

All Barty, playing two hours before Williams, had to do was emerge victorious from her match against Alison Riske, an unheralded American veteran who was ranked 55th and had never made so much as a dent at a tennis Grand Slam event.

But Riske had other plans.

A 29-year-old Pittsburgh native with an energetic personality and a punishing game on grass, Riske upset Barty, 3-6, 6-2, 6-3, setting the tone for long stretches with hard, flat groundstrokes and aggressive volleys.