A crash for George Russell and a water leak triggered two red flags as the final F1 track action of 2023 got underway in Abu Dhabi.
The two red flags followed a delay as the medical helicopter had not arrived for the planned 09:00 start time.
Once running did get underway, Russell was the first driver on track, followed by Red Bull's Sergio Perez.
After 35 minutes, the red flag was shown for a water leak between Turns 13 and 14 – the short stretch underneath the hotel.
Felipe Drugovich was fastest at the time with a 1:26.265s; the Aston Martin driver having logged 13 laps to sit 0.005s clear of Perez.
It was a sizeable leak and came from the left-hand side of the circuit. Officials used blowers and a truck to clean it up, resulting in a 24-minute delay.
After restarting, Perez moved to the top of the timesheets by 0.2 seconds over Piastri, with a best of 1:25.724 seconds.
At the other end of the timesheets, Lance Stroll was without a lap time to his name despite having been on track several times – cycling back to the pits on each occasion.
Teams had a wealth of tyres at their disposal, with five different compounds and 10 sets available for the primary car over the day.
That bank comprised one set of C1 rubber, one set of C2, two C5s, and three each of the C3 and C4.
The Young Driver car, however, had two sets of the C3 and C5 rubber and four sets of C4.
F1 will abandon the C0 next season, reverting to five compounds from which three are selected for any given weekend.
The tyre compounds are also set to remain the same, at least for the start of 2024.
A second red flag was shown when Russell found the wall at Turn 6 with an hour remaining in the morning session.
According to Mercedes, an issue with the car led to the crash in which the front-right corner was heavily damaged.
The incident happened on Russell's 59th lap, just over a full race distance in Yas Marina, and triggered a 28-minute delay.
After four hours of action, Sainz fastest for Ferrari on a 1:24.799s with 69 laps to his name.
Pato O'Ward has been busiest, completing 74 tours, while Ayumu Iwasa managed just 46 laps for Scuderia AlphaTauri.
Several driver changes are set to take place for this afternoon, with Logan Sargeant replacing Alex Albon at Williams, Fernando Alonso in for Stroll, Charles Leclerc for Sainz, Yuki Tsunoda for Daniel Ricciardo, and Zak O'Sullivan in place of Franco Colapinto in the second Williams.
Four hours of running remain in Abu Dhabi, track action set to conclude at 18:00 local time.