If Williams were taking their own route on Fridays as they normally do then the potential might be there to mix it with the Ferraris tomorrow. However, I think the Ferraris will be out of sight on Sunday, like the last couple of races.
Williams aren't anywhere at all, they have zero chance this year of doing anything interesting outside of reliability/damage. They've barely developed, have a meh car and no real plan going forwards.
I think midway through last year Williams made a step, likely adjusting to the engine more completely, since then, nada. Low drag no downforce design, they've lacked downforce since day one of testing last year and still do today. Honestly I'm incredibly disappointed in them, smaller budget, sure, other small budget teams are taking FAR bigger chassis risks and have more down force. I feel more like Lotus, TR and RBR are more likely to go by Williams than Williams gain even a bit on the guys in front of them.
Merc vs Ferrari pace, slightly strange numbers, a second behind the other two on a heavy fuel on slower tires is about right but not showing the gap they showed last week. Rosberg's run for soft tires looked slow on any fuel by comparison. Slower driver, maybe, maybe just Merc aiming for a higher delta to just check the drop off. Maybe they want to know stint length when slower and if it's not good enough they might try a burn around like a bat out of hell three stop race.... what they should have done from the start in Malaysia.
The other option, maybe more likely, is that just the older the engines guess they more they are engine saving and the more they will rely on computer models to estimate tire life on faster stints. Run around at 1:40's, get data, plug it in, get estimates for life life doing 1:38's.