Fish advanced with a 6-3, 6-4 decision over Belgium's Xavier Malisse. He goes on to face 34-year-old German Tommy Haas, who continued a recent surge with a 6-1, 6-2 rout of countryman Tobias Kamke in a 62-minute match.
Querrey beat third-seeded Kevin Anderson of South Africa, 6-4, 6-4 at William H.G. FitzGerald Tennis Center.
His next opponent will be Alexandr Dolgopolov of Ukraine, who posted a 7-6, 6-4 triumph over 2002 Washington champion James Blake of the United States.