Washington, DC, USA, 1 August 2014 | AFP/AAP

Lleyton Hewitt has been knocked out of the ATP Citi Open in Washington DC by big-serving Wimbledon semifinalist Milos Raonic.

The Canadian blasted 27 aces on his way to winning the tight third-round clash 7-6(1) 7-6(3) on Thursday at the $US1.9 million ($A2 million) hardcourt tune-up for the US Open.

“I did a lot of good things, especially the way I fought through the important games,” said Raonic, the tournament’s second seed.

Raonic, who also won his opener over American Jack Sock in two tiebreakers, will face US giant-killer Steve Johnson for a spot in the last four.

Johnson dumped ATP aces leader Ivo Karlovic 3-6 7-6(4) 7-6(7) despite 27 aces by the Croatian ninth seed a day after beating US fifth seed John Isner, who ranks second in aces and fired 29 past Johnson.

“He couldn’t have had any better preparation to face me,” said Raonic, who ranks third in ATP aces.

And Johnson could face South African seventh seed Kevin Anderson, who ranks sixth on the season aces list, in the semifinals.

Anderson downed Tunisia’s Malek Jaziri 6-3 6-4 to reach a quarterfinal match with American left-hander Donald Young, who outlasted Uzbek Denis Istomin 6-3 3-6 6-3.

French sixth seed Richard Gasquet, coming off a right arm injury that saw him withdraw last week at Atlanta, beat American Tim Smyczek 6-3 6-2.

Czech top seed Tomas Berdych was ousted from the tournament after falling 6-2 6-4 to Canada’s Vasek Pospisil, who now faces Santiago Giraldo.