Johnson, who triumphed on clay in Houston earlier this year, relied on his big serve to win 83 percent of first-serve points and wrap up the final in the only U.S.-based grasscourt ATP event in just under two hours.
With the win, Johnson brought an end to world number 161 Ramkumar's bid to become the first player from India to win an ATP singles title since mentor Leander Paes prevailed in Newport 20 years ago.
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Ramkumar, who also has a big serve as well as an appetite for attacking the net, made an impressive run to the finals in an upset-filled tournament that saw only two seeded players reach the quarter-finals.
But defeating Johnson, who did not drop more than three games in a set en route to the final, proved too big a challenge for the Chennai native.
The match was played three hours earlier than originally scheduled due to a possible storm in the area.
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