Sebastian Vettel topped the timesheets once again at Jerez today, while a water leak signals that Mercedes still isn't entirely bullet proof.

Sterling work by the Lotus crew meant that, contrary to expectations, Lotus was able to get out on track for some serious lappery today.

However, when Pastor Maldonado brought out the red flags for the first, and only, time - leading to much wicked speculation on Twitter - following a "drivetrain related issue", it signalled another late night for the Enstone crew.

All was going well for Mercedes, Hamilton setting a good pace and continuing to amass the mileage, until around 13:50, when an innocent enough incident turned out to be something more.

"Lewis stopped just short of the marks in pit lane (by a few hundred metres...), car back in the garage for inspection," tweeted Mercedes. However, a subsequent tweet revealed: "Minor water system problem the reason for Lewis switching off in pit lane entry. Expecting to have him back out in a jiffy."

Two hours later, rivals who had winced as the German team looked set to add another 150 laps its tally breathed a sigh of relief when it revealed: "That'll be us for today - investigations revealed a water leak from a failed part. The crew are now working hard to get the car ready for a return to action tomorrow."

Hamilton's frustration however, was but nothing compared to Jenson Button as McLaren continues to suffer. The session was almost three hours old before the Briton made a dent on the timesheets, albeit 33s off the pace. Though he reappeared in the final stages of the session, as the rain began to fall, his lap count for the day equalled teammate Fernando Alonso's from yesterday, six!

With the last fifty minutes of the session hit by rain, rookies Max Verstappen and Felipe Nasr took the opportunity for some serious wet weather driving, the Dutch youngster, in particular, looking more than competent.

It was a day of pure frustration for Red Bull, the team side-lined following a minor off early in the session which saw Daniil Kvyat damage his front wing. Unfortunately the team didn't have a spare, and as a result the few remaining laps (9) that the Russian covered over the course of the day were without a front wing.

So, another strong day for Vettel and also Valtteri Bottas, interestingly the only drivers on duty both days.

The eight drivers in action today covered 465 laps, compared to the 450 completed yesterday by just seven.

"It's too soon to give a verdict on the true level of this car," said Vettel, who added another 89 trouble-free laps to his tally, "especially as we can't make a comparison with the other teams, who can be running different programmes. However, I'd say we have a good basis to work from. We did a lot of running, even in the rain, when we tried the new intermediate tyres, which worked well, getting up to temperature despite the cold: that's a definite step forward."

"The applause in the garage at the end of the session?" he added, referring to something which had the paddock wondering. "Well, everyone was happy we’d even managed to run in the wet with nothing bad happening. The boys have got enough to do in the garage as it is…"

"A bit like yesterday, in that even if we did a good number of laps, you can never claim to have done enough kilometres," said James Allison. "However, I agree with Sebastian on the fact that, for the moment, we have a good basis."

Sauber enjoyed another successful day, debutant Felipe Nasr completing 88 laps as he concentrated on becoming familiar with the car and its systems. During the entire day, the car ran absolutely trouble-free.

"It was a great day," said the Brazilian. "I was able to complete a lot of laps, and I was happy with everything I saw. I was getting used to the systems, the procedures and the characteristics of the car. It was definitely a good start to understand where we are now. It was important for me to get used to this new environment, but after such a long day, with both dry and wet conditions, I was able to adapt to all the situations. Now we can start working on the details."

"It was another positive day," added Giampaolo Dall'Ara, the Swiss team's Head of Track Engineering. "We were able to run through our programme again without any major issues. It was Felipe's first day in the car, which was also good for us to get to know each other and work together. In the morning we went through some systems checks, set-up work and some normal scans of various kinds. In the afternoon we evaluated different tyre compounds and power unit settings.

"The only unforeseen element of the day was the weather, because we only expected dry conditions. But we had no drama there, as we adapteded our programme, and Felipe drove in the wet at the end of the session, which gave us some extra information. We seem to have a good basis to work from."

"We have had another good day," said Valtteri Bottas, who completed 61 laps on his way to posting the third fastest time. "We managed to get through all of our programme and it was only the final run that we were compromised due to the rain. The morning was dedicated to obtaining aero data, then this afternoon we focused on the longer runs, making set-up changes and analysing the results. We are learning all the time and at the moment I am really pleased with what I have felt when in the car.

"We learnt a lot yesterday and have continued to build on that today," added Rod Nelson, Chief Test & Support Engineer. "We started aero performance based work today, continuing to move through the test schedule we have set ourselves. We have had no issues with the car today, so have made good progress."