John Isner has come through a tense tie-break decider against Lleyton Hewitt to reach the Atlanta Open final after a hard-fought semi-final clash.

The American looked to have taken control by claiming a tight first set, with a pair of ferocious forehands earning him a valuable break and a 6-4 advantage.

Both players exchanged breaks early in the second set, but it was Hewitt who found consistency at the baseline to make his second break count and level the match.

Hewitt had three break points to take a crucial lead at 3-3 in the deciding set, but Isner fought back behind a huge first serve to survive.

Neither player could find the upper hand, meaning that a third-set tie-breaker would decide who progressed to the final.

Despite a spirited effort from the Aussie veteran, Isner made the breaker count, claiming match point at the first time of asking for a 6-4 4-6 7-6(5) victory.

The 28-year-old will now meet the winner of today's second semi-final between Ryan Harrison and Kevin Anderson for the Atlanta Open title.