Muchova returned to action in June after missing 10 months because of surgery on her right wrist.

Wednesday’s match was delayed because of rain right before Noskova served for the second set with the Czechs ahead 5-4. When play resumed, they took that set, then dominated the tiebreaker.

“Honestly, sometimes 10-point tiebreakers are a little unlucky,” Pegula said. “They played pretty much the perfect tiebreaker.”

The 19-year-old Noskova closed out the victory with a volley winner.

Her biggest achievement to date came at the Australian Open in January, when she beat Swiatek in the third round. That made Noskova the first teenager to beat a No. 1-ranked woman at Melbourne Park since 1999.

“I was just standing there, letting her play,” Muchova said with a laugh about her partner, “and that’s how we won.”