Ferrari emphatically put three days of low-key performances with their new F2012 behind them to storm to the top of the timesheets on the final day of the first test at Jerez.
The aggressively-designed new challenger had struggled on the opening three days of the 'winter season' and on Thursday night the team's technical director Pat Fry admitted concern with where the team currently were at with the car.
But on Friday Alonso wasted little time in lighting up the timesheets in Spain with the local star using the soft Pirelli tyres to vault to a 1:19.289 inside the first hour before returning to lower the benchmark still further to 1.18.877 - the second fastest time for a 2012 car all week.
It was an impressive retort to the doubts that had gathered earlier in the week. However, the day was not without significant glitch for the team, with Alonso forced, shortly after setting what proved to be his insurmountable time, to return to the pits due to what is reported to have been another hydraulics fault with the F2012. It thus proved to be a bittersweet day for the team, with Alonso only able to put relatively few laps on the board in total - 39.
But Fernando wasn't the only high-profile name to be left kicking his heels in the paddock with Sebastian Vettel's RB8 suffering its first reliability problem of the winter after just two installation laps.
An electrical fault kept the German driver in the Red Bull garage until mid-afternoon, but, on the positive side he quickly moved his way up the order when he did return to the track before eventually finishing in third place - 0.7s adrift of Alonso.
Sandwiching the two championship favourites was rookie Frenchman Jean-Eric Vergne, who capped a promising week for the new Toro Rosso STR7 by pipping Vettel in the sister Red Bull team.
Lewis Hamilton, meanwhile, completed what appears to have been a highly productive - and reliable - week for McLaren with the fourth fastest time, having for the second successive day all-but matched the pace being set by RBR and Vettel.
The former World Champion put a further 86 laps on the MP4-27.
Day three pacesetter Romain Grosjean slipped to fifth for Lotus, but the Frenchman's benchmark time from Thursday still ended up unbeaten by the end of the four-day test to give the Enstone outfit much encouragement over their new E20's potential.
Kamui Kobayashi was a further tenth of a second adrift in the new Sauber C31, the Japanese another driver force to spend some time on the sidelines after a hydraulic leak mid-way through the afternoon left him stranded on the circuit.
Once the problem was fixed though, he returned late in the day for a further 18 laps to take his overall tally up to 76.
After Jules Bianchi's Thursday morning crash had scuppered his scheduled debut in the new VJM05, Force India's Nico Hulkenberg finally got his first run in the car on Friday and duly completed 90 laps to take seventh.
Behind Bruno Senna - who put a mammoth 125 laps on the new Williams-Renault FW34 - was another driver making a belated first appearance of the week, Caterham's Jarno Trulli.
The veteran Italian, whose winter has been dogged by rumours that he may be replace in the team's race line-up, got his first taste of the team's new KERS system and the new CT01 proved reliable enough for he too to comfortably clear a century of laps.
The pre-season testing schedule resumes at Barcelona's Circuit de Catalunya on February 21.
Day four times:
1) Alonso 1:18.877 39 laps
2) Vergne 1:19.597 80
3) Vettel 1:19.606 50
4) Hamilton 1:19.640 86
5) Grosjean 1:19.729 95
6) Kobayashi 1:19.834 76
7) Hulkenberg 1:19.977 90
8)Senna 1:20.132 125
9) Trulli 1:22.198 117
