By Martyn Herman PARIS, May 31 (Reuters) – If young Americans like Ryan Harrison and Jack Sock are to keep climbing the rankings they will need plenty of the staying power compatriot John Isner demonstrated once again on Friday. The 28-year-old Isner, who won the longest tennis match of all time at Wimbledon in 2010, delved deep into his fuel reserves to fight off Harrison, 21, from two sets down to reach the third round of the French Open. …

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