Williams is one team member down in Malaysia this weekend.
Paul Bray, the British team's electrician, was rushed to a Kuala Lumpur hospital on Friday where he had his appendix removed.
It was a typically hot and sticky start to qualifying day on Saturday, and by the start of first practice at 1pm the previously blue skies had clouded over.
It is not expected to rain at least during the first part of the Saturday practice session.
However, it is almost certain that it will rain at around 4pm, the time scheduled for qualifying.
If the rain is too heavy, the FIA has told the teams that qualifying could take place early on Sunday morning.