(Deletes extraneous UPDATE 1 in headline)
* Murray overcomes back spasm to progress at French Open
* Mathieu takes record 76 games to beat Isner
* Nadal moves smoothly into third round
Mental strength and physical endurance won the day at the French Open on Thursday when fourth seed Andy Murray battled through the agony of a back problem and Paul-Henri Mathieu staggered through a record 76 games to win in the second round.
Fourth-seed Murray, who said he came within a few points of retiring, needed three bouts of massage from the trainer on court before beating Finn Jarkko Nieminen 1-6 6-4 6-1 6-2.
Mathieu, ranked 261 in the world after 15 months out of action while his left leg was surgically rebuilt to correct a knee problem, looked shell-shocked after taking five hours 41 minutes to knock out American 10th seed John Isner.
His 6-7 6-4 6-4 3-6 18-16 victory on the Philippe Chatrier showcourt, played with a broken toe, was a Roland Garros record in terms of games played.
While others struggled, defending champion Rafa Nadal sped through his second-round match, beating Uzbekistan's Denis Istomin 6-2 6-2 6-0, and fifth seed Jo-Wilfried Tsonga needed only an hour to complete a win over German Cedrik-Marcel Stebe after a rain delay overnight.
The leading women of the day also progressed safely, with 2011 champion Li Na sailing past Frenchwoman Stephanie Foretz Gacon 6-0 6-2 and Italy's Francesca Schiavone, the runner-up last year and winner in 2010, recovering from some early serving problems to beat Bulgarian Tsvetana Pironkova 2-6 6-3 6-1.
Murray and Mathieu were the crowd favourites, though, for their fortitude.
Scot Murray, a semi-finalist here last year, confessed he had considered not playing when he got out of bed in agony with a back spasm.
"I woke up this morning... couldn't put any weight on my left leg," he told a news conference.
His personal physiotherapist, who had already treated his client for a back injury earlier this month that put him out of the Madrid Masters, counselled that playing would not make matters worse and so Murray played.
He grimaced through the first set, gritted his teeth through the massages, moved woodenly, then took to staying on his feet during the changeovers.
As the match progressed, Murray's back began to ease and he started hitting winners, just as Nieminen's game went to pieces.
When he gave away a breakpoint in the seventh game of the final set, the Finn dashed his racket to the ground and stamped on it. A double fault followed, and Murray was 5-2 up.
"I just couldn't believe that I had won when I finished the match," Murray, who now faces Colombian Santiago Giraldo, told a news conference.
Mathieu, 30, had three matchpoints at 11-10 in the final set but Isner, already in the record books for playing 183 games when he beat France's Nicolas Mahut 70-68 in the fifth set at Wimbledon two years ago, fought back to prolong the match as the sun began to set over Roland Garros.
"At the end of the match I didn't even believe I had won," said Mathieu after, 13 games later, Isner put a forehand wide on matchpoint number seven.
Mathieu's win was all the more remarkable as he came into Roland Garros with a broken toe.
"Four days before this tournament I broke a toe against a bench. After the match I have two bruises on two nails of my toes, because (they) hit the shoe for six hours. It's just a bruise. There are worse things than that."
Isner was not interested in talking about another entry in the record books. "It stinks that I lost," he said. "I am just going to go home, I don't want to think about tennis right now."
While Mathieu and Isner took two hours 28 minutes just for their fifth set, Nadal spent less than two hours on Suzanne Lenglen court.
The Spaniard, aiming to win a record seventh French Open title, has dropped only nine games in his first two rounds and, worryingly for his opponents, said he was in even better form mentally than last year.
"I gained confidence, and when you gain confidence you feel a lot better and things are more easy for you," said Nadal, who can hope to have an equally comfortable third-round match against Argentine qualifier Eduardo Schwank.
Tsonga was happy that the first rain of the week had stopped his match on Wednesday, children's day at Roland Garros, as he had found conditions too distracting.
"I had difficulties focusing," Tsonga said after completing a 6-2 4-6 6-2 6-1 win. "There was a lot of noise, and I was getting a bit crazy. So I think it was a good idea the match was stopped yesterday and I could start again in far better conditions for me."
His compatriot Virginie Razzano's time in the spotlight came to an end. The world number 111, who knocked out 13-times grand-slam champion Serena Williams in a thrilling first-round match, lost tamely to Dutchwoman Arantxa Rus 6-3 7-6.
Second seed Maria Sharapova had to wait for her chance to go through the second round. Due on Philippe Chatrier court after Mathieu v Isner, Sharapova's meeting with Japanese player Ayumi Morita was eventually postponed until Friday.