Top seed John Isner was eliminated from the Citi Open quarter-finals as Gael Monfils reached the final four.
Isner, a three-time runner-up in Washington, went down to fellow American Steve Johnson in straight sets but did not leave without a fight on Friday.
The fifth-seeded Johnson eventually came out on top 7-6 (9-7) 7-6 (17-15) in a thrilling match that lasted just under two hours.
Johnson will now face Ivo Karlovic in the semi-finals of the ATP 500 event after the Croatian was too good for Jack Sock 7-6 (7-4) 7-6 (8-6).
The big-serving Karlovic hit 26 aces on his way to the straight-sets win without needing to convert a break point.
Karlovic and Johnson have faced each other four times in the past, with the latter holding the better record via three victories.
Meanwhile, 2011 runner-up Monfils battled to a 6-4 3-6 6-1 win over American Sam Querrey.
The French second seed withstood a fightback from Querrey and was in dazzling form in the third set to take out the match.
Monfils will meet Alexander Zverev in the other semi-final after the German's 6-1 6-3 victory against Benoit Paire.
Zverev took out the first set in just 17 minutes - the quickest all tournament - as the uninspiring Paire drew boos from the crowd for his lacklustre performance.
Wildcard Zverev needed a bit longer in the second but was too strong for Paire in the end.