Officials hoped to complete much of the schedule, although rain remained in the forecast for the rest of the day.
Six second-round singles matches and 16 doubles matches were postponed. Among those delayed until Friday were Nadal's match against Horacio Zeballos and Williams' match against Julia Goerges.
During the delay, the players' cafeteria and lounge areas in the main stadium were packed with players and their entourages, and there were long lines at concession stands and souvenir shops. Several hundred fans sat under umbrellas on center court watching a video replay of last year's final between Roger Federer and Robin Soderling.
"Rain continues to fall at RG," American doubles specialist Bob Bryan tweeted four hours into the delay. "I'm on the floor of the lockeroom in my bed of towels playing Angry Birds. Anyone have 3 stars on all levels?"
Rain came with the tournament already behind schedule following three delays Wednesday, the longest of which lasted 90 minutes. Four men's second-round matches were suspended because of darkness Wednesday night, and four women's matches were postponed.
The only washout of an entire day's schedule on record at Roland Garros occurred May 30, 2000. Even so, the French tennis federation has been pushing for a roof and has studied the possibility of moving the tournament, with a site next to the Versailles castle among the locations being considered.
A project announced last year to build a new court with a retractable roof less than 500 yards from the current center court was blocked because of political issues.
Wimbledon added a retractable roof last year atop a stadium that opened in the 1920s, and the Australian Open has retractable roofs over two courts. But the French and U.S. Open must wait when it rains.