Defending champion Alexander Zverev came from a set down to beat Kei Nishikori and reach the semi-finals of the Citi Open on Friday.
The 21-year-old German overcame the Japanese in straight sets to reach the final last year, but he was made to work harder this time around to progress in the ATP 500 event in Washington, D.C.
The top seed dropped his serve in the sixth game to give up the only break of the opening set.
But he held throughout the second to get back on track, ultimately winning 3-6 6-1 6-4 after breaking Nishikori twice in each of the second and third sets.
The world number three will face Stefanos Tsitsipas for a place in the decider after the Greek upset Belgian third seed David Goffin 6-3 6-4.
Promising young Australian Alex de Minaur received a walkover route into the semi-finals after former world number one Andy Murray withdrew due to exhaustion.
Murray's match against Romanian Marius Copil did not finish until 03:00 local time on Friday morning and the three-time grand slam winner has also pulled out of the Rogers Cup as he eases himself back to full fitness following hip surgery in January.
De Minaur meanwhile registered an eye-catching victory over South Korean Hyeon Chung in the previous round.
He will face either Denis Kudla or Andrey Rublev in the last four. Their quarter-final was suspended due to rain late on Friday and will be completed on Saturday.
