Jack Sock pulled off an impressive upset, while Kevin Anderson, Alexander Zverev and Kei Nishikori also advanced to the ATP Citi Open semi-finals on Friday.

Sock's serve was on song in the 7-5 6-4 win over Milos Raonic, with a 59 per cent first serve rate, winning 80 per cent of those points.

Although the Canadian third seed won 57 per cent of points on Sock's second serve, he simply did not get enough opportunities on it to make an impact.

After falling behind 1-3 early on, the American eighth seed was able to grind the match out and secure his first semi-final berth in five appearances at Washington.

"He's played more big matches than me. Over time I've played a lot of big matches and seen that serve and game style a few more times, so I was able to scrap out, play a lot of good defensive points and get through," Sock said afterwards.

He will face 15th seeded South African Anderson, who took three sets to dispatch Yuki Bhambri, 6-4 4-6 6-3.

Despite defeating top seed Dominic Thiem earlier in the week, it was not much easier against the Indian qualifier, whose first serve rate was at a gargantuan 70 per cent.

Anderson converted on both break point opportunities he had, including Bhambri's opening service game of the third, and held on.

Meanwhile, Zverev needed less than an hour to defeat Daniil Medvedev 6-2 6-4.

Nishikori did it tough against big-serving American youngster, Tommy Paul, eventually winning 3-6 7-6 6-4.

Brimming with confidence after beating Gilles Muller and Lucas Pouille, Paul raced to a 4-1 lead in the first set.

The Japanese second seed only showed a sense of urgency when Paul was serving for the match at 5-4 in the second.

Saving three match points, Nishikori then steadied the ship with his defensive game and secured the crucial break in the final set.