WASHINGTON, DC, USA - American teenager Catherine McNally knocked out the last remaining seed at the Citi Open on Friday.

The 17-year-old upset No.4 seed Hsieh Su-wei in the nation's capital in straight sets to reach her first-ever WTA semifinal, 6-4, 6-3. 

With No.5 seed Lesia Tsurenko's loss to Zarina Diyas in the second round, Hsieh was the lone seeded player to make it out of the first two rounds this week, but the veteran's craft was undone by strong returning and heavy topspin from the American teenager.

"I wanted her to win so badly because I just thought it would be an amazing experience to play against her," McNally said.

"I know how she plays. I think I was just totally prepared for everything. That was super helpful. I think that's what helped me today.

"She hits a lot of shots that most players don't hit, two hands on both sides. She'll hit some unbelievable angles. I was just ready for that. My coach helped me prepare for that. I think I did a really good job."

McNally broke Hsieh's serve five times in the match, and ran off a string of games at the end of each set to score her best-ever win in terms of ranking over the World No.31.

From 3-2 down in the opener, McNally won four of the final five games, before she later ran off the last four games of the second set to seal the upset in straight sets.

With a run to the doubles final alongside fellow teenager Coco Gauff, this week in the nation's capital has been the tour-level coming out party for the World No.150: in the semifinals, she'll faced Italy's Camila Giorgi, who defeated Diyas, 6-3, 6-2.

Read more: Gauff and McNally into Citi Open doubles final: 'Our games mesh together really well'

"I'm really excited. She's a really good player. She's done very well, had a long career," McNally said.

"I'm just excited to play my game, mix in the slice, see how she handles everything that I can give to her heavy ball. I'm just looking forward to the matchup.

"Coco's run [at Wimbledon] was obviously unbelievable. I'm super proud of her. I think that just kind of put a fire in my belly that showed me I can do the same thing.

"I didn't put any expectations on myself this week. I mean, I'm not really surprising myself because I know that I'm capable of doing this. So I'm just super happy that I'm going out there, enjoying it, executing my game really well,"