Lewis Hamilton has endorsed Ferrari as the favourites at this stage of pre-season testing and suspects the first race in Melbourne will be a close one.
The Mercedes driver finished up fourth quickest, but was more than half a second slower than Ferrari's Sebastian Vettel on the same tyre – the soft compound.
Hamilton admitted his run in the morning wasn't indicative of Mercedes' pace as the team struggled with some issues, but nonetheless he suspects Ferrari could be favourites come the first race of the 2017 season.
"It didn't feel spectacular this morning," Hamilton told the BBC. "But we had some issues with tyre temps and with some floor damage so I sacrificed some of my time so Valtteri could run this afternoon.
"I think Ferrari are possibly the favourites. We can't take our eyes off them. They've done such a good job at the moment and Red Bull look like they've gone quite quick as well today so we shall see over the next days and most importantly in the next weeks.
"I think it's going to be close at the first race, that's for sure."
Williams also impressed on the fifth day, with Felipe Massa setting the quickest time on the supersoft compound.
Hamilton is hopeful of a Williams resurgence: "I hope, particularly Williams, they come back. That would be amazing. Williams have had a serious dip for so many years.
"They've been coming back strong the last few years and I really hope they are fighting with us – that would be amazing for the sport to see more top teams fighting and particularly iconic teams."
