Janowicz, the No. 24 seed, got past Jurgen Melzer 3-6, 7-6 (7-2), 6-4, 4-6, 6-4. Kubot defeated Adrian Mannarino 4-6, 6-3, 3-6, 6-3, 6-4.
Wednesday's match between the two will mark the first Grand Slam quarterfinal appearance for both. It will also be the first time the two countrymen will have played each other in an ATP match.
Janowicz had just two more points overall (150-148) than Melzer but had the only break opportunity of the fifth set, which he converted to advance.
Kubot, No. 126 in the world, had 10 of his 26 overall aces in the fifth set. He faced just one break point and saved it. Mannarino made Kubot work for the only break in the final set, saving 10 break points but losing one, which allowed Kubot to reach the quarters.
Among players more familiar with major tournament quarterfinals, world No. 1 Novak Djokovic won a fourth-round match for a 17th consecutive Grand Slam event and No. 2 Andy Murray attained his sixth consecutive Wimbledon quarterfinal.
Djokovic defeated 13th-seeded Tommy Haas 6-1, 6-4 7-6 (7-4). Djokovic saw his serve broken twice but still won about 70 percent of the points on serve. He posted 40 winners against 16 unforced errors and won nearly half (50-of-107) of the points when Haas served.
Murray was a 6-4, 7-6 (7-5), 6-1 winner over No. 20 Mikhail Youzhny. Murray did lose his serve twice but had only 16 unforced errors. He also broke Youzhny's serve five times.
He next faces Fernando Verdasco, who came out ahead 6-4, 6-4, 6-4 over Kenny de Schepper Monday. A veteran of 40 major tournaments, Verdasco has made the quarterfinals just twice before (U.S. Open 2009, '10).
Fourth-seeded David Ferrer stopped Ivan Dodig 6-7 (3-7), 7-6 (8-6), 6-1, 6-1 and No. 8-seeded Juan Martin del Potro beat No. 23 Andreas Seppi 6-4, 7-6 (7-2), 6-3. Ferrer and del Potro are paired against each other in the next round.
Djokovic's quarterfinal opponent is seventh-seeded Tomas Berdych, who beat Bernard Tomic 7-6 (7-4), 6-7 (5-7), 6-4, 6-4 on Monday.