This semi-final comes after Roddick and Isner won their respective quarter-finals against Juan Monaco and Marcos Baghdatis.
Roddick, the top seed, eased through to the last four as he beat seventh-seeded Monaco 6-1 6-4 in just 66 minutes.
The 28-year-old did not face a break point, surrendering only nine points on serve in total, and converted three of his five chances on Monaco's serve.
Fourth seed Isner, in contrast, had a far tougher contest.
The world number 28 fought back from a set down to defeat the former world number Baghdatis 1-6 6-3 6-4 with the pair set to meet again next week in the first round of the US Open.
Roddick will take a 3-1 lead into the match and he said: "You can have a game plan, but it's a lot tougher to execute.
"A lot of times it's out of your hands and it's completely different from the matches I've played so far.
"It's just whoever can scrape a return back on a big point and hope that something good happens.
"The biggest thing is you've just got to try and take care of your own serve.
"I don't often go into a match with the second-best serve, but that will be the case tomorrow."
In the other semi-final Robin Haase will meet Julien Benneteau.
Haase knocked out Alexandr Dolgopolov despite the third seed producing 15 aces.
The Dutch player enjoyed more all-round consistency although both players produced a number of double faults - five for Haase and six for his opponent.
Haase, though, had more success on his serve as he won 6-4 6-4 in an hour.
Benneteau produced an upset when he knocked out Sergiy Stakhovsky who is ranked 72 places above the Frenchman at 41.
Stakhovsky served up 12 aces - three more than Benneteau - but also nine double faults as his opponent claimed a 3-6 7-6 (11/9) 6-2 victory.