Washington, DC, USA, 23 July 2016 | tennis.com.au

American wildcard Jessica Pegula has caused the upset of the Citi Open in Washington DC, beating top seed Sam Stosur in straight sets to advance to the semifinals.

Pegula, ranked No.173, defeated the Australian 7-6(4) 6-3 in just shy of 90 minutes and goes on to face the winner of the match between Lauren Davis and Camila Giorgi.

Players were inseparable in the opening set; in a battle pitting Stosur’s power against Pegula’s gritty retrieving abilities, both held serve throughout.

The ensuing tiebreak was one Stosur will be happy to forget; she lead 4-1 only to drop six straight points to lose it.

“I don’t know how (I won that),” Pegula said.

“There were a couple of long points. She missed a couple of easy shots. But I just hung with it and tried to serve well.”

Leading 0-30 in the sixth game of the second set, Stosur appeared on track for the first service break of the match, yet a rash of errors proved her undoing.

Emboldened, Pegula then broke serve in the eighth game to push ahead 5-3.

The increasingly aggressive wildcard showed no nerves closing out the match, sealing victory with a forehand winner.

Stosur will next head to Montreal for the Rogers Cup.