Paris: Top seed and five-time winner Rafael Nadal escaped one of the biggest upsets in tennis history after fighting back from a set down to beat America's John Isner 6-4, 6-7 (2/7), 6-7 (2/7), 6-2, 6-4 at the French Open on Tuesday.

The Spain tennis star, bidding for a record-equalling sixth Roland Garros title, had never previously dropped a set in the tournament's first round but came close to a huge shock against the big-serving world number 39.

The defending champion Nadal, whose clay-court invulnerability was punctured by recent losses to Novak Djokovic in the finals of the Madrid and Rome Masters tournaments, will confront countryman Pablo Andujar in round two.

Isner saved four break points at 3-3 in the first set before conceding the break with a backhand that landed marginally wide.

Nadal closed out the set and then broke Isner in the opening game of the second set, only for the American to break back in game eight when Nadal sent a forehand wide.

Isner took a 5-2 lead in the subsequent tie-break and when a wayward return from Nadal gifted the American four set points, Isner grabbed the opportunity to level the match with a lusty serve that left the champion floundering.

It was the first time Nadal had ever dropped a set in the opening round at Roland Garros and the first time in the first round of a Grand Slam since the 2007 US Open, but worse was to follow.

The 24-year-old was not patrolling the clay with anything like his usual authority and spurned two set points in the third set allowed Isner to drag him into another tie-break.

Once again, Isner assembled a 5-2 lead on the back of some trademark booming serves and when Nadal needlessly netted a backhand the American pounced, slamming home a forehand return to take the set.

Nadal stopped the rot by breaking in the third game of the fourth set, before levelling the match to set up a deciding fifth set that began amid fervent excitement on the central court.

Isner proved his stamina by outlasting France's Nicolas Mahut in a world record 11hr 5min marathon in the Wimbledon first round last year, but he quickly found himself a break down in the decider.

Unable to make inroads on Nadal's serve, he survived one match point on his own serve but then came off second-best in an epic rally to hand Nadal another match point that the Spaniard converted with barely concealed relief.