The Finn clocked a 1:37.231, three seconds slower than the pole time on Saturday, to top the timesheet after the first four hours of running.
Sebastian Vettel ended the morning in second place, but the morning session was not without its troubles for the German.
He span off at the first corner of the Yas Marina circuit, bringing out a brief red flag period as the marshals had to repair the barriers.
The Mexican was the busiest of the ten drivers in the morning session, with 61 tours of the Yas Marina complex completed.
The Finn could only manage 39 laps in the morning session, and was second-slowest out of the ten runners.
He could only manage 18 laps, and was five seconds off the pace of the Mercedes.
This test is being conducted for the teams to have some running on the 2019 tyre compounds, whilst also being able to use the 2018 sets to compare and contrast between them.
Despite this, not much information is given out from trackside, so we are unable to say which tyres the drivers were on when they put their best laps in.
- Valtteri Bottas – Mercedes – 1:37.231 (54 laps)
- Sebastian Vettel – Ferrari – 0.354s (31)
- Max Verstappen – Red Bull – 0.716s (53)
- Sergio Perez – Force India – 0.745s (61)
- Nico Hulkenberg – Renaut – 1.558s (50)
- George Russell – Williams – 2.281s (39)
- Sean Gelael – Toro Rosso – 3.204s (45)
- Lando Norris – McLaren – 3.284s (48)
- Kimi Raikkonen – Sauber – 3.601s (39)
- Pietro Fittipaldi – Haas – 5.060s (18)