Pegula served out the final game to love to win against GauffJessica Pegula beat fellow American and doubles partner Coco Gauff 6-2 5-7 7-5 in the Canadian Open quarter-finals.
Pegula, 29, broke her 19-year-old opponent twice on the way to claiming the first set but Gauff edged the second to level.
Gauff was not helped by nine double faults and Pegula earned the decisive break at 5-5 in the third set before serving out for the match.
"It's always tough to play your doubles partner," said Pegula.
"It just came down to the wire and who was going to compete better."
Pegula will play Iga Swiatek or Danielle Collins in the semi-finals.
Liudmila Samsonova beat world number two Aryna Sabalenka 7-6 (7-2) 4-6 6-3 in their delayed third round match.
She will play Belinda Bencic in the last eight after the Swiss came back from a set down to secure a 6-7 (3-7) 6-3 6-1 victory against Petra Kvitova in another match which had been moved to Friday.
