Fifth seed Robin Soderling was given an intensive first-round workout as he overcame American Ryan Harrison 6-1 6-7 (5-7) 6-3 7-5 at the French Open.
Harrison, 19, only made the main draw after Benjamin Becker dropped out, but impressed again after making the last 16 at Indian Wells in March.
Eleventh seed Nicolas Almagro was the highest-profile upset as Poland's Lukasz Kubot won through in five sets.
Last year's semi-finalist Jurgen Melzer breezed past Andreas Beck 6-3 6-4 6-2.
Spaniard Almagro served for the match in the fourth set but was unable to resist as world number 122 Kubot turned the tables and surged back to a 3-6 2-6 7-6 (7-3) 7-6 (7-5) 6-4 victory.
Austrian eigth seed Melzer showed no sign of the back injury that forced his retirement from the first round of the Rome Masters as he overcame Beck in an hour and 58 minutes.
Spain's Fernando Verdasco, who has made the last 16 in his last four visits to Roland Garros, came through a tricky opening encounter with Argentina's Juan Monaco 6-2 7-5 4-6 6-4.
Germany's Florian Mayer and Ukraine's Alexandr Dolgopolov, seeded 20th and 21st respectively, made their way through to the second round with comfortable wins over Rainer Schuettler and Igor Kunitsyn.
Sam Querry, the tournament's 24th seed, also advanced with a 3-6 6-1 6-2 6-4 win over Philipp Kohlschreiber
Querry will take on former world number three Ivan Ljubicic next after the Croat beat India's Somdev Devvarman 6-4 6-3 6-4.
After France's Arnaud Clement gave the home crowd more to cheer with as he made up 85 ranking places to beat Italy's Filippo Volandri 6-3 1-6 7-6 (9-7) 6-4.
Clement's countrymen and 18th seed Gilles Simon proved too good for American Michael Russell as he won through 6-3 4-6 6-1 6-0.
But fellow Frenchman Nicolas Mahut failed to spring a surprise as South African 32nd seed Kevin Anderson triumphed 6-4 6-4 6-7 (6-8) 6-3 in their match.