Tommy Haas continued the run of form which has taken him from outside the top 200 to world No. 36 as he beat Argentina's Leonardo Mayer 6-4, 7-5 at the ATP/WTA event in Washington.
The 34-year-old German fourth seed is playing some of the best tennis of his career after finally regaining full potential following a hip operation last year.
He is playing for keeps this week in the US capital as he tries to bury the disappointment of missing the London Olympics after the German federation refused to nominate him for wild card tournament consideration.
Haas lined up a quarter-final clash with compatriot Tobias Kamke, who earned a revenge win over Marinko Matosevic, 6-4, 6-3. The Australian had beaten Kamke last week in Los Angeles.
Haas, who defeated Roger Federer to win at Halle in June, improved his 2012 record to 21-9 as he bids for a 14th career trophy.
He advanced over Mayer in just over 90 minutes, hammering eight aces including two in the 11th game of the second set.
The veteran, who came to Washington for the first time 15 years ago, then broke a game later to clinch the win.
"It was a good win, a very hard-fought match," said Haas. "He has a big serve and there was a lot of guessing on my part out there.
"We had tough, humid conditions and of course it's hot. But that's the beauty of tennis, you have to deal with it all.
"Playing Tobias in an all-German match will be good. I'm looking forward to it."
Kamke advanced into the last eight on his third match point against Matosevic.
Second seed Alexandr Dolgopolov continued his winning tournament debut week, beating Spain's Ruben Ramirez Hidalgo 6-1, 6-3.
Crowd-pleaser James Blake, champion a decade ago in his prime, beat Swiss qualifier Marco Chiudinelli 6-2, 6-4 to earn his first quarter-final since Stockholm last November.
The world number 116 winner is finally fit after a knee operation in 2011. He won his third match of the season, now lining up against Dolgopolov.
In WTA women's play in the newly combined event, Russian top seed Anastasia Pavlyuchenkova advanced over Melinda Czink 6-2, 2-6, 6-3.
American seeds Vania King and Coco Vandeweghe will clash after world number four King beat French qualifier Aravane Rezai 6-2, 6-2 while Vandeweghe, a Stanford finalist against Serena Williams, defeated Irina Falconi 6-2, 7-6 (7/5).
"The weather was not as hot for the match today, but I'm very glad to go through. I'm playing well so far," Vandeweghe said.
Junior Wimbledon winner Eugenie Bouchard of Canada upset eighth seed Olga Govortsova of Belarus 1-6, 6-0, 6-3, returning to the scene of her first main draw win a year ago.
Slovakia's Jana Cepelova beat Edina Gallovits-Hall of Romania 6-3, 6-4 while Magdalena Rybarikova defeated Greek Eleni Daniilidou 6-3, 7-5.