"I was serving well and I just hung in there," Ramkumar, who won 6-4 7-5 on the Newport grass, told ATPWorldTour.com.
"I didn't have the greatest first set on return, but I found a groove. He's a really tricky opponent, especially on these courts. I'm pretty happy to get through."
At world number 161, the 23-year-old is the third-lowest ranked player to reach a tour-level final this year. The last player from India to advance to a championship match on the ATP World Tour was Somdev Devvarman in Johannesburg in 2011.
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Ramkumar, who has dropped one set all week on the way to his first ATP World Tour final, fired seven aces to the American's four in the 90-minute match.
Third seed Johnson needed just 68 minutes to beat Granollers, winning 88 percent of his first serve points and taking advantage of six double faults from the Spaniard.
Earlier on Saturday, 1991 Wimbledon champion Michael Stich and Czech Helena Sukova, who won 14 Grand Slam doubles and mixed doubles crowns, were inducted into the International Tennis Hall of Fame.
(Reporting by Gene Cherry in Raleigh, North Carolina; Editing by Peter Rutherford)
