Mother Nature smiled down on the Spanish circuit as the Formula One drivers geared up for the final day - and only the second on a dry track. With the session barely underway, Kamui Kobayashi, holding the P1 slot, caused the first stoppage as the Japanese driver came to a halt right after the chicane, bringing out the red flags.
His car was soon cleared away and the action resumed with Tonio Liuzzi taking the P1 slot off the Sauber driver before he lost out to Robert Kubica.
The Renault driver set the fastest time of this year's Jerez tests, a 1:19.114, which was 0.155s up on the previous best that had been set by Mark Webber on Friday. But the Pole then came under attack from Button.
Having languished near the back of the timesheets, the reigning World Champ moved up to second place in a brief three-lap stint before heading back to the pits.
A few minutes later he ventured back out, clocking a blistering 1:18.87 on the second lap on his latest three-lap run to take the lead - and the fastest time for the year - off Kubica.
With Button, Kubica and Liuzzi leading the way, an uninterrupted two hours of track time was finally interrupted.
Mark Webber was responsible for the day's second red flag as his RB6 came to a halt near Turn 4. 14 minutes later, testing was back up and running - and the clouds were moving in.
With a healthy crowd watching on, Button did a 1:19 flat to effectively hold the two fastest times for the week.
Two-and-a-half-hours of clean running ended with Button on top, followed by Kubica, who was 0.243s adrift. Liuzzi was third, ahead of Nico Rosberg and Fernando Alonso.
While some of the drivers focused on short runs during the morning, others embarked on longer stints with Rosberg averaging 1:21.9, Alonso 1:21.9, Hulkenberg 1:23.8, Kubica 1:24 and Jarno Trulli 1:25.9. Those times ranged over ten to 20 lap runs.
Saturday marked Trulli's maiden voyage for Lotus and although the Italian was slowest of all, 5.410s off the pace, it was an admirable showing as he completed 65 laps.
"Jarno getting more comfortable in the car and picking up pace," reported tech boss Mike Gascoyne. "Reliability looking good. Changing brakes as Jarno not happy with brake bite. Otherwise good progress."
As for fellow newcomers, Virgin Racing, Timo Glock managed just 14 laps as an undisclosed problem left him stranded in the pits.
Unofficial Times
1. J. Button McLaren MP4-25 1:18.871 26 laps
2. R. Kubica Renault R30 1:19.114 +0.243 54 laps
3. V. Liuzzi Force India VJM-03 1:19.650 +0.779 43 laps
4. N. Rosberg Mercedes GP W01 1:20.150 +1.279 62 laps
5. F. Alonso Ferrari F10 1:20.436 +1.565 61 laps
6. K. Kobayashi Sauber C29 1:20.751 +1.880 45 laps
7. J. Alguersuari Toro Rosso STR5 1:21.053 +2.182 66 laps
8. M. Webber Red Bull Racing RB6 1:21.194 +2.323 43 laps
9. N. Hülkenberg Williams FW32 1:21.919 +3.048 93 laps
10. T. Glock Virgin Racing VR-01 1:22.856 +3.985 14 laps
11. J. Trulli Lotus F1 T127 1:24.641 +5.770 65 laps
Source: Planet-F1
