Despite starting the season with five consecutive 1-2s, Mercedes is looking to further improve the W10, and with Bottas leading the way in terms of pace and only slightly off the mark in terms of laps completed, the German outfit's domination of the sport is likely to continue for some time yet.
While this morning was relatively trouble-free, there were two red flags this afternoon, the first when Nick Yellowly crashed in the Alfa Romeo, and the second when Pierre Gasly stopped on track.
It was a heavy crash for Ilott, the Briton losing the Alfa Romeo and hitting the barriers at Turn 3. Though he was unhurt in the incident, the car was too badly damaged to play any further part in proceedings.
While Bottas posted his best time on the softest rubber in Pirelli's 2019 range, the C5, second quickest was Charles Leclerc, who though 1.422s off the Finn's pace, set his best time on the C2 rubber.
Daniil Kvyat, who for many was 'man of the race' on Sunday was third quickest on the C4, ahead of Hulkenberg and Gasly who were both on C3s.
Ferrari and Racing Point had two drivers on duty, Leclerc and Nick Yellowly doing regular testing, while Sebastian Vettel and Sergio Perez were on duty for Pirelli as the Italian manufacturer trials its 2020 compounds.
Mercedes focused on expanding its understanding of the W10 as well as completing some aero data collection.
"Today's test day has been really good," said Bottas. "We completed a lot of laps and learned a lot.
"Our programme focused on the tyres and setup. We collected lots of data over the race weekend in similar conditions to what we had today in terms of temperature and wind, so it was nice to do some setup comparisons.
"There were some new mechanical test items on the car too which were interesting to try out. Everyone did a really good job after a long weekend, so it's been another successful day."
"We've had a trouble free day and got through most of the programme," added Andrew Shovlin. "The number of test days is so limited now that you always end up with a very busy programme and it's important that everything arrives on time with it being our last chance to test this season.
"We've had a number of developments on the chassis and Power Unit side which we can hopefully bring to the race car over the next few races. We've also done some work on setup for the softer compounds that will be appearing in Monaco and Montreal, along with some longer run work.
"We have Nikita (Mazepin) in the car tomorrow, he's worked with us over the past few months but this will be his first run in the W10 and we have a busy day of testing lined up for him."
At Ferrari, Sebastian Vettel did 58 laps in the morning, setting a best time of 1:18.425, while Charles Leclerc worked on a comparison and development programme. The Monegasque driver took to the track trying some updates and running comparison tests to continue gathering data and information relating to the car's handling. He did 66 laps, the best in a time of 1:17.322, using the C3 tyres.
Both SF90s were on track again in the afternoon, Vettel completed a further 75 laps, making a total of 133, equivalent to 619 kilometres, while Leclerc did a further 66, for a total of 132. Vettel's best time was the one he set in the morning, whiles Leclerc got down to a 1:16.933 on the C2 compound tyres.
Tomorrow, Leclerc will switch to working on the Pirelli programme, while replacing him will be Antonio Fuoco. The Italian is one of a group of drivers working in the simulator and this will be his third track test for the team after having already completed a total of 856 laps at the wheel of a Ferrari. The first was in Spielberg in 2015 and then here in Barcelona in 2016.
"It was a positive day, overall," said Leclerc. "We completed a total of two race distances and collected a lot of data which we will analyse to understand our car even better. We are pushing to continue making improvements and I look forward to being back on track tomorrow."
"It was a productive first day as we covered all of our programme and test items we had planned," said Daniil Kvyat. "The team worked flawlessly, and we had really reliable and consistent running which is what we wanted. We can put a big tick in the box for this test and hopefully we will learn a lot from it, so we can bring some new ideas to races in the near future."
"The good weather and faultless running from the car and Daniil allowed us to complete our programme," added Guillaume Dezoteux, Toro Rosso's Head of Vehicle Performance. "We were focused on aero data gathering in the morning, following the introduction of a new package during the race weekend just passed. We then worked on different setup directions on the C3 tyre compound to optimize and further understand our car and Daniil's requirements. In the afternoon, we continued our setup work on the C3 and performed some long runs for tyre optimization using the harder C2 tyre compound. We completed the day with a couple of short runs on the C4 tyre compound, which is one step softer than the soft tyre we used here during Qualifying.
"It is great to be back testing as it offers more opportunities to learn and secure clear answers about the car. Race weekends are often more difficult with limited running, variable tyre allocation, track evolution, and sometimes drivers learning a new track layout. It is always more difficult to be brave and try significantly different configurations. Now we will review all the data with our colleagues in Faenza and Bicester, and prepare the car for Alex's run plan for the final day of 2019 in-season testing tomorrow."
"Using a testing PU fitted on Monday, Kvyat completed a high mileage, allowing us to acquire plenty of data," said Masamitsu Motohashi (Honda F1 Deputy Technical Director). "This is particularly useful as we have the figures from only a few days ago over the Spanish Grand Prix weekend to use as a benchmark. Some of our work was aimed at tuning the PU elements for the next race in Monaco. We also carried out routine tests as well as doing some long-term development work and we went through our programme as scheduled."
Pierre Gasly was back in action for Red Bull, the French driver enjoying a strong outing at the Montmelo track, posting a total of 118 laps as the team continued development work on the RB15.
"It was a busy day in the car and we managed to get through lots of test items," said Gasly. "Today was all about trying to establish a direction for the races to come and also to extract a bit more performance from our package. We had what looks like a transmission issue right near the end of the session, which is being looked at now, but we still managed to get a lot of laps done."
"There are always things you'd like to try with the car that just aren't possible on a race weekend," added Simon Rennie, the team's Group Leader Simulator Engineering, "so based on what we learned during the race weekend, the plan today was to look at a variety of set-up concepts that we believe will aid general performance across the rest of the season.
"That meant a pretty intense schedule for Pierre and for everyone in the garage as we worked through those changes, but we managed to get a lot done and we acquired some good data. In the morning Pierre ran on the C2 compound that was nominated as the medium tyre last weekend.
"Then, in the afternoon, he spent a short period on the C1 compound for some aero runs before focusing on the C3 tyre up until the stoppage. We're looking into the cause of that now, but we'll certainly be up and running again for tomorrow's session."
"Today has been a positive day for us as we managed to complete the whole programme planned for my morning session in the car," said Carlos Sainz, who completed 64 laps. "We went through some set-up items that we had doubts about after the weekend, and found some interesting results to explain why we weren't completely happy with our qualifying and race performance. We also validated some new aero parts that we brought here this weekend. We've got the data we need to take back home. That will help us understand what we did well this weekend and what we need to work on for Monaco. Overall, I'm happy with the work done."
"Today was a decent day," added Lando Norris. "We got through everything we wanted to do today which was positive. We also went through a few things to confirm the results from the weekend before starting some preparation ahead of Monaco. We didn't do a huge number of laps this afternoon but from what we did, we got everything we needed and managed to learn a lot."
"Today has been a useful day of testing because we could complete our programme without any issues or delays," said performance director, Andrea Stella. "The test coming after the race gives us the opportunity to make some direct comparisons with the race weekend.
"In particular, we tested a few things on car set-up that helped us understand the behaviour of the car in qualifying and in the race, and we also acquired some useful information for the coming races.
"We conducted some aerodynamic tests to learn more about the aero package that we took to the Spanish Grand Prix, in addition to some further aerodynamic developments that we are considering for future introduction. We also gathered some information which will be very useful for the development of the car."
Check out our Tuesday gallery from Barcelona, here.