In a tremendous atmosphere on Suzanne Lenglen, Altmaier clinched a 6-7 (0) 7-6 (7) 1-6 7-6 (4) 7-5 second-round victory after five hours and 26 minutes – the fifth longest match in the tournament’s history – having saved two match points when Sinner served for the win in the fourth set.
A tense finale saw Altmaier broken when he served for the match for the first time at 5-4 in the decider and then pegged back from 40-0 at 6-5.
Italian Sinner had three chances to force a deciding tie-break, and chucked his racket to the clay in anger after missing the final one, before Altmaier, ranked 79, took his fifth chance with an ace.
The German, who lost to Sinner in five sets at the US Open, sat in his chair sobbing afterwards, saying: “I love clay and I love the crowd and everything. The emotions were crazy.”
Altmaier, who reached the fourth round on his Roland Garros debut in 2020, next faces Grigor Dimitrov after the Bulgarian’s more straightforward 7-6 (4) 6-3 6-4 victory over Emil Ruusuvuori.
An open bottom half is now even more so, with fourth seed Casper Ruud the highest-ranked player remaining.
The Norwegian appears to be playing himself into form at the right time after a difficult season so far, and said after a 6-3 6-2 4-6 7-5 victory over Giulio Zeppieri: “It was tough to try to finish the match, and it was a two tough sets, but I’m happy to win one of them and be through to the third round.”
Twelfth seed Frances Tiafoe fought back from a set down to defeat Aslan Karatsev 3-6 6-3 7-5 6-2 but 18th seed Alex De Minaur fell in straight sets to Argentina’s Tomas Etcheverry.
Etcheverry next meets 15th seed Borna Coric, who won another five setter against Pedro Cachin, and Thiago Seyboth Wild followed up his upset of second seed Daniil Medvedev by beating Guido Pella 6-3 3-6 6-4 6-3.