John Isner of the US hits a return to Serbia's Novak Djokovic during their Davis Cup quarter-final tennis match in Boise, Idaho, in this April 5, 2013 file photo. [Photo/Agencies]

PARIS - 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.

While nowhere near as long as the 76-game marathon Isner endured when losing to Paul-Henri Mathieu here a year ago and not in the same league as his 11-hour five-minute match against Nicolas Mahut that entered tennis folklore, Friday's 5-7 6-7 (7) 6-3 6-1 8-6 win was a bruising battle.

Harrison is yet to back up his early promise and languishes 87th in the rankings, still looking for his first ATP title.

Friday's failure to finish off 19th seed Isner will cast fresh doubts over his ability to win the big matches, having now lost his last nine in grand slams to seeded players.

However, he could take inspiration from Isner who had lost his previous six five-set battles, including at every grand slam tournament last year, but keeps coming back for more.

"Winning a five‑set match is extremely satisfying ... every single one," Isner told reporters.

"I played well in 2012 but that really was a tough pill to swallow for me. This one helps and I really do think it will help me going forward."

Harrison has never got past the second round at a grand slam, although in mitigation he has suffered with bad draws.

"To say I'm not frustrated by it is a lie," he said of his grand slam record. "I want to be in the later rounds, second week, and doing bigger things than I'm doing now.

"I competed my butt off out there. I came close and we're talking a backhand crosscourt and a tape from being up in the fifth.

"Hopefully I go to Wimbledon and don't have to play Roger (Federer) or Novak (Djokovic) or one of those guys first round."

Qualifier Sock, 20, is still to crack the top-100 and was given a lesson by battle-hardened German Tommy Haas on Friday.

"It was a great learning experience. I kind of know now what I need to do to get to the next level," said Sock, who hails from Nebraska like former world number one Andy Roddick, after his 7-6 (3) 6-2 7-5 defeat.