Novak Djokovic survived the first serious test of his French Open credentials by fighting back from two sets to one down to beat Diego Schwartzman.
The defending champion had reached round three without dropping a set despite not playing his best.
But he was given a real fright by Argentinian Schwartzman before coming through 5-7 7-5 3-6 6-1 6-1.
Schwartzman, ranked 41, has never beaten a top-10 player but he has tested the likes of Rafael Nadal and Kei Nishikori this season.
The 24-year-old is only 5ft 7in tall but his shots pack a real punch and he shocked Djokovic by coming from a break down to take the opening set.
Djokovic, with coach Andre Agassi sat impassively in the stands, looked to have turned things around when he claimed the second set but Schwartzman was not finished.
At times, the second seed resembled the player whose dominance of the game was total only 12 months ago.
But there were too many unforced errors - 55 in total - and at costly times.
After Schwartzman broke for 6-5, Djokovic had four chances to force a tie-break but could not take any of them, the crowd feeding off the Argentinian's energy.
But maintaining such intensity over five sets is extremely difficult and one of the reasons the format favours the top players.
Djokovic raced into a 4-0 lead at the start of the fourth set, with his only blip coming when he dropped serve in a fiery fifth game.
Umpire Carlos Ramos penalised Djokovic a first serve for a time violation and then gave him another warning for bad language.
Djokovic argued his case while the crowd whistled and jeered, but it did not disrupt his momentum.
Schwartzman had given all he had and the champion lives to fight another day.
Elsewhere, Nadal laid down a marker as he chases his 10th French Open title with his most one-sided ever victory at Roland Garros.
The fourth seed and favourite to lift the Coupe des Mousquetaires next Sunday allowed Georgia's Nikoloz Basilashvili just a single game in a 6-0 6-1 6-0 victory.
It is the first time in a completed match at the tournament that Nadal has only lost one game.
The biggest cheer of the match came when Basilashvili, a little-known 25-year-old ranked 63, finally won a game at 6-0 5-0.
But that proved to be as good as it got for the Georgian, whose final tally of five winners and 34 unforced errors rather told its own story.
Nadal hit 27 winners, most of them off his lasso forehand, which is in ominously good shape.
Basilashvili looked like he might be saved by the weather as thunder rumbled overhead late in the third set.
Nadal afforded himself a chuckle as Basilashvili wasted a bit of time by changing his racket.
But the rain did not arrive and he was put out of his misery after an hour and a half, with Nadal moving through to a fourth-round clash with fellow Spaniard Roberto Bautista Agut.