Top-seeded Samantha Stosur has lost to world No.173 Jessica Pegula in the quarter-finals of the Citi Open in Washington.

Stosur lost 7-6 (4) 6-3 on Friday (Saturday AEST) after the 2011 US Open champion led 4-1 in the opening tiebreaker before she lost six consecutive points to drop that set.

The only break of the match came in the next-to-last-game, when Pegula went up 5-3, then served out the biggest victory of her career.

The 22-year-old’s win over the world No.14 moved Pegula into her first WTA semi-final.

Her parents own the NFL’s Buffalo Bills and NHL’s Buffalo Sabres.

On Saturday (Sunday AEST), Pegula will face either Lauren Davis or Camila Giorgi for a berth in the final.

The other semi-final at the hard-court tournament will be No.6-seeded Yulia Putintseva against No.7 Yanina Wickmayer.

Wickmayer beat No.4 Kristina Mladenovic 4-6 6-3 6-4, while Putintseva defeated Risa Ozaki 6-4 6-2.