Former world champion Jenson Button overcame a morning spent largely in the pits by setting the fastest time on the first day of Formula One testing a month in Spain ahead of the opening Grand Prix in Australia.
The 33-year-old Englishman — world champion in 2009 — overcame problems with his McLaren as he roared back in the afternoon session to record a fastest time of 1min 18.861sec over 34 laps.
"It was an up and down day," said Button.
"It is best to have this type of problem at the beginning of testing rather than the first day.
"We've had the problem, and I hope that we can work well this week.
"But be careful because all these times don't tell us anything, even those on the last day of testing (3 March in Barcelona) don't really reveal much," added the 228 Grand Prix veteran.
Red Bull's Australian star Mark Webber was second fastest after completing 73 laps in the team's new car, which they hope will deliver a fourth successive constructors' title.
"A time of 1:18.8 at Jerez on the first day with a new car isn't bad at all," said Webber, whose team-mate three-time world champion Sebastian Vettel didn't take to the track.
Lotus's unpredictable French driver Romain Grosjean was third fastest, while Scottish driver Paul di Resta was in fourth in the new model Force India car.
Webber's compatriot Daniel Ricciardo set the pace in the morning session in the new Toro Rosso car but ended up fifth-fastest.
Button's former McLaren team-mate and compatriot Lewis Hamilton didn't take to the track in his Mercedes, whose day's work had been cut short after German driver Nico Rosberg's car caught fire after just 14 laps because of an electrical fault.
Minnows Marussia had an inauspicious day as, hours after unveiling their new car, British rookie Max Chilton crashed after exiting the track — he did manage to complete 29 laps but was way off the pace of the 10 other drivers with his best time over six seconds slower than Button's.
Marussia are likely to be trading blows at the back of the grid with Caterham this season after the withdrawal of last season's tail-enders, HRT.
1. Jenson Button (GBR/McLaren-Mercedes) 1min 18.861sec 37 laps
2. Mark Webber (AUS/Red Bull Renault) 1:19.709 73
3. Romain Grosjean (FRA/Lotus-Renault) 1:19.796 54
4. Paul di Resta (GBR/Force India-Mercedes) 1:20.343 89
5. Daniel Ricciardo (AUS/Toro Rosso-Ferrari) 1:20.401 70
6. Felipe Massa (BRA/Ferrari) 1:20.536 64
7. Nico Hulkenberg (GER/Sauber-Ferrari) 1:20.699 79
8. Nico Rosberg (GER/Mercedes) 1:20.846 14
9. Pastor Maldonado (VEN/Williams-Renault) 1:20.864 84
10. Giedo van der Garde (NED/Caterham-Renault) 1:21.915 64
11. Max Chilton (GBR/Marussia-Cosworth) 1:24.176 29