John Isner progressed into round three of the Miami Masters in Key Biscayne, but was made to work for his victory over fellow American Donald Young.
The 10th seed, who made it to the semi-finals in Indian Wells last week before being beaten by Novak Djokovic, lost the first set on a tie-break (7-5) as he struggled to find his best tennis.
However, the 28-year-old bounced back by breaking Young midway through the second set and was able to keep his nerve to level the match.
Isner then broke his opponent again early in the third set as the world number 10 started to look more assured on court after an indifferent opening.
He was able to hold out to set up a third-round tie against Nicolas Almagro, who beat American Sam Querrey in straight sets.
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