French Open tournament director Guy Forget said it is a "necessity" for a roof to be built at Roland Garros after a washout on Monday, but is confident the clay-court grand slam will finish on time.

No play was possible on day nine due to heavy rain in Paris, with top seeds Novak Djokovic and Serena Williams seeing their fourth-round matches put back.

The French Open is the only one of the four major tournaments without a roof over any of the courts and Forget stressed the importance of addressing that issue.

"I think right now there is proof that it's a necessity and we have to do it. We can't be like that for many years." he said.

"Although we have very good hope that it will be done [a roof added] by 2020, I hope everything will go on in between now and then so that no one can just stop the process, because once again, it is a necessity.

"I'm a bit annoyed today, to say the least, although I kind of smile sometime with the players. It's hard.

"Tomorrow looks better than today, so I'm positive we will be able to play, which is a good start. We knew today was going to be horrible. It went beyond what we imagined. That's why we sent the players back so early.

"We are not that far back on schedule. It's still reason to be optimistic and we are. We are confident, and hopefully we will be able to play tomorrow as much as we can, and we will speed it up, start earlier."

Forget does not expect the finals, scheduled to be played this weekend, will be postponed as a result of the inclement weather as it stands.

"That's looking at the really dark scenario. We're not that bad yet. But eventually if they have to play two matches in a row, they will do it." he added.