The scores were 3-6, 1-6, 6-3, 4-6.
The match, Santoro's record-equaling 20th French Open appearance before a partisan crowd at Roland Garros, was suspended Tuesday because of darkness.
Santoro is retiring at season's end.
When play was called Tuesday, Rochus led 5-3 in the fourth set.
He needed just eight minutes Wednesday to complete his victory and advance to the second round.
"I wanted to have played a tight match," Santoro said on the ATP Web site. "I wanted emotions. I think I served well (but) I wish I had won the fourth set to be able to play the fifth."
The Tahiti-born Santoro has played in a men's tennis record 67 grand slam tournaments and his best showing was advancing to the fourth round at the French in 1991 and 2001 and the Australian Open in 1999.