Carlos Sainz says Williams's hopes of building on a giant-killing start to Formula 1's Australian Grand Prix weekend rest on him unlocking something he does not know how to get yet.
Williams's 2025 signing raised eyebrows in first practice in Melbourne as he ended the session second quickest behind McLaren's Lando Norris.
He then made an equally strong start to FP2, mixing it with the frontrunners during the early runs on medium tyres before falling back to 11th as everyone made a switch to soft runs and leapfrogged him.
Sainz feels that the weakness shown on the soft C5 tyre confirmed what he suspected from testing: that the Williams car is strong but needs to be able to extract more pace on the softest tyre compounds.
"It's what I said in an interview, I think it was in the Bahrain test, where to unlock the potential on the soft tyre, it's the key about this car," he said. "It's what I don't know yet how to do, and today proves it.
"I was very quick on mediums and very competitive also in FP1, but as soon as everyone turned up in FP2, and I went to find the laptime on the soft, I clearly struggled to find the lap time on that compound.
"Tomorrow we will have four or five softs in quali, and hopefully that will give me a chance to unlock it. That's where there is all the work to do."
Sainz's team-mate Alex Albon said that the way the car did not make progress on the soft was unusual.
"We need to understand why, as it doesn’t feel right, and it’s important to get on top of that ahead of qualifying,” said Albon, who set an identical time to Sainz later in FP2.
"On the medium tyre, I feel we would comfortably be in Q3, but the softs have changed a bit from last year and we are just trying to understand them."
Lack of experience
Williams was not alone in struggling to find extra pace when switching to the soft tyre.
Mercedes' George Russell, who had decent long-run pace on the hards, did not find much gain from the C5 in second practice as he suffered from overheating by the end of each push lap.
"Every time we had the medium or hard tyre on, we were right in the top two on the timesheets," he said. "Every lap felt good, felt confident. When we put the soft on, we didn't go much quicker.
"There is clearly a bit of pace in the car and it's just getting the most out of the tyres, so we need to understand why that is."
F1 tyre supplier Pirelli said that there was no obvious explanation as to why some teams were struggling more with the softs than others.
Instead, chief engineer Simone Berra believed the issue was purely down to a lack of experience teams had with the new-specification soft.
This was exaggerated by the fact that Albert Park is a track where it is difficult to nail perfect tyre temperatures for both the front and rear axles throughout a lap.
"The compounds are new for the teams, and they are gaining experience lap after lap," explained Berra. "So I think it's just a matter of trying to understand it, and put the tyres in the correct working window.
"Here, you have the rear axle ready quite soon, but the front axle sometimes gives you hesitation in the first couple of corners.
"So it's always a matter of having both axles in the correct window, not just a matter of the C5 compound being in the window.
"I think that they have to work a little bit on the balance of the car and on the preparation of the lap to find the peak of grip.
"We know that C5 is peaky, even if the new C5 compared to the old one is less peaky because we increased a little bit the working range.
"We do think that it should be easy in the future for the teams to extract a bit more from the C5 compared to the past."
A tight grid
But while unlocking more pace from the soft could help Sainz fight closer the front, he still thinks that the top four teams hold an advantage, especially over long runs.
"On high fuel you can see there's quite a clear step from ourselves and maybe the Ferraris and McLarens, and even Mercedes and Red Bull,” he said. “But hopefully we are not too far [behind] and we can keep fighting the midfield.
"But the field is extremely tight. I saw the [Racing Bulls] doing some very fast laps, also a bit out of nowhere. It seems honestly that if you put together a good lap, anyone can be in Q3."
