Lewis Hamilton ended this week's Barcelona test at the top of the timesheets having edged former team-mate Jenson Button on Friday.
As the rain came down at the start of the fourth and final day for this week at the Circuit de Catalunya, Hamilton held the early advantage with a 1:38.
As a dry line began to form, Hamilton lowered the benchmark to a 1:31.920 before Felipe Massa - back on track after an early spin - joined the tussle at the front, clocking a 1:31.298.
The Ferrari driver continued lapping, shaving full seconds off his time to get it down to a 1:27.351. That, though, was short-lived as ten minutes later Hamilton came flying through with a 1:25.929.
Force India hopeful Jules Bianchi got involved in the mix as he momentarily took the top spot before losing out to Jenson Button with a 1:25.313.
Hamilton hit back with a 1:23.282 while Button dropped his time to get within half a second of his former team-mate.
The rain returned putting an end to any improvements and it continued to fall heavily drenching the track and ensuring Hamilton finished top of the timesheet as most drivers spent the afternoon either practice pit stops or lapping 15 to 20s off their previous best times.
Second place went to Button, 0.351s adrift, while Jean-Eric Vergne claimed third place in his Toro Rosso ahead of Bianchi and Esteban Gutierrez.
Caterham's Giedo van der Garde was sixth quickest on a day when timing counted for everything. He was, however, four seconds short of Hamilton's best.
Neither Williams driver, Valtteri Bottas nor Pastor Maldonado, set a lap time as the Grove-based team opted to focus on pit stop practices in the wet conditions.
Meanwhile, there were no fewer than seven red flag situations on the final day, which cost the drivers over an hour of track time.
The first was early in the morning when Massa spun out when it began to rain which followed by the first of two of van der Garde. Gutierrez also caused two red flags, both the result of the Sauber rookie sliding off into the gravel.
The other two belonged to Mark Webber, who stopped out on track just after Campsa, and Vergne, who joined the list of those getting beached in the gravel.
The final stoppages, which came in the last 30 minutes, of play saw Hamilton grind to a halt between Turns 6 and 7 while Bianchi stopped near the pit exit.
01. Lewis Hamilton Mercedes 1:23.282 52 laps
02. Jenson Button McLaren 1:23.633 +0.351 70 laps
03. Jean-Eric Vergne Toro Rosso 1:24.071 +0.789 80 laps
04. Esteban Gutierrez Sauber 1:25.239 +1.957 61 laps
05. Jules Bianchi Force India 1:25.732 +2.450 96 laps
06. Giedo van der Garde Caterham 1:27.429 +4.147 50 laps
07. Felipe Massa Ferrari 1:27.563 +4.281 80 laps
08. Mark Webber Red Bull 1:27.616 +4.334 64 laps
09. Max Chilton Marussia 1:29.902 +6.620 51 laps
10. Romain Grosjean Lotus 1:34.800 +11.518 41 laps
11. Valtteri Bottas Williams no time 23 laps
12. Pastor Maldonado Williams no time 13 laps
Day Four Gallery From Barcelona