Sebastian Vettel and Red Bull were quickest by a comfortable half-second margin as Formula 1 2013 winter testing resumed at Barcelona on Tuesday morning.
Even by testing standards, the initial session featured little activity at first, with most teams remaining coy and focusing on shakedowns and aero tests for the opening part of the morning.
Once the more serious runs began, times came down quickly and first place changed hands repeatedly.
On the morning that launched its new FW35, Williams spelt a long spell on top with Pastor Maldonado.
Nico Rosberg then put Mercedes ahead, before Sergio Perez edged in front after a long pause to reset in the McLaren garage.
As he tried the Ferrari F138 for the first time, Fernando Alonso deposed Perez for a few minutes only for Paul di Resta's Force India to snatch first place away again and Vettel to then vault both.
Kimi Raikkonen then became the first man to lap under 1m24s in the morning with a 1m23.5s. Alonso closed to within half a tenth of the Lotus, but then Vettel took charge going into the final half hour.
A series of improvements got the Red bull to 1m23.046s, 0.525s clear of Raikkonen.
Alonso, Maldonado and Toro Rosso's Daniel Ricciardo were next up.
Perez completed the session in sixth, followed by di Resta, Nico Rosberg and Esteban Gutierrez, with Charles Pic and Max Chilton bringing up the rear in the Caterham and Marussia.
The only stoppage of the morning came when Gutierrez's Sauber deliberately ran out of fuel at Turn 7.
Pos Driver Team Time Laps 1. Sebastian Vettel Red Bull 1m23.046s 25 2. Kimi Raikkonen Lotus 1m23.571s + 0.525s 19 3. Fernando Alonso Ferrari 1m23.626s + 0.580s 44 4. Pastor Maldonado Williams 1m23.733s + 0.687s 47 5. Daniel Ricciardo Toro Rosso 1m23.884s + 0.838s 44 6. Sergio Perez McLaren 1m24.124s + 1.078s 22 7. Paul di Resta Force India 1m24.144s + 1.098s 47 8. Nico Rosberg Mercedes 1m24.520s + 1.474s 14 9. Esteban Gutierrez Sauber 1m25.136s + 2.090s 38 10. Charles Pic Caterham 1m27.534s + 4.488s 40 11. Max Chilton Marussia 1m28.068s + 5.022s 36