Andy Murray's hopes of a first French Open title remain intact after he pulled off another great escape against Radek Stepanek.
The world number two recovered to win a match from two sets down for the eighth time in his career, beating 37-year-old qualifier Stepanek 3-6 3-6 6-0 6-3 7-5.
Murray, whose last first-round exit at a grand slam was more than eight years ago, lost the first two sets when the match began on Monday but had turned things around and led 4-2 in the fourth when darkness brought an end to play.
He had no trouble polishing off that set but the fifth was hugely tense and at one point Stepanek was two points away from victory.
But Murray held firm and then took advantage of the first drop in his opponent's level to clinch victory after three hours and 41 minutes.
The 29-year-old, who now plays Frenchman Mathias Bourgue, paid tribute to Stepanek, saying: "It's unbelievable what he's doing.
"He had an extremely bad injury last year and still at 37 coming out and fighting like that, playing that way, it's unbelievable. I don't expect to be doing that myself at that age. I'm just glad I managed to get through.
"He's always been extremely difficult to play. I wasn't able to dictate many of the points, I wasn't in a great rhythm, and that's credit to him and the way that he played. I fought extremely hard today and I'll get a chance to play again tomorrow."
Murray had the momentum on Monday having won 10 of the last 12 games but the overnight break allowed Stepanek to regroup and refresh.
The Czech, looking to become the oldest man to win a grand slam singles match since Jimmy Connors at the US Open in 1992, had played brilliantly for two sets on Monday.
He has caused Murray plenty of problems in the past, beating him at Queen's Club in 2014 and taking a set off him at the Madrid Open earlier this month.
At least the scoreline gave Murray a little cushion that he would not have had had he won the fourth set on Monday, setting up a one-set shoot-out.
It looked like he might need it when Stepanek created two break points in the opening game but this time Murray saved them despite grumbling at the umpire about the crowd and the overhead camera.
He had a set point on Stepanek's serve, which the Czech saved, but Murray took his third chance and walked back to his chair with fist aloft.
Stepanek is well known for his ability to wind up opponents both with his canny play and on-court antics, and after withstanding Murray pressure in the opening game of the decider he whipped up the crowd.
Murray gestured towards his box in frustration but the conditions at least were not quite as heavy, although it remained cool and overcast, and he was able to dictate more of the points.
The Scot was able to threaten the Stepanek serve but not break, with the world number 128 doing a superb job of hanging on, pushing Murray's level of anxiety skyward.
When Stepanek held for 5-4, Murray had to serve to stay in the tournament. The Czech threw everything at his opponent as the pair traded fizzing groundstrokes but deuce was as close as he got.
The effort of that appeared to affect Stepanek in his next service game as for the first time he buckled and Murray broke.
That left the Scot serving for the match. Still it was not straightforward and Murray double-faulted on his first match point, but on his second Stepanek netted a volley and a relieved Murray held his arms in the air.