Alberto Moncayo, riding in his first full grand prix season, splashed to the fastest lap time at the end of a soaking wet second Friday 125cc practice session at Estoril in Portugal.
The Andalucia Cajasol Aprilia rider snatched the top spot from WTR San Marino's Johann Zarco on his very last lap.
Zarco had previously dominated the 40-minute session, despite a small fall, and finished just over one second behind Moncayo.
Nico Terol was third quickest, 1.297sec from Moncayo, and was the only one of the three title contenders to stay on two-wheels.
Championship leader Marc Marquez, who starts the penultimate round with a 12-point lead over Terol and 17-point advantage over Pol Espargaro, crashed from his Red Bull Ajo Derbi after losing the front on his first fast lap.
Marquez was unhurt, but the bike was damaged, forcing him to sit out the rest of the session. The 17-year-old was classified just 30th as a result.
Espargaro then had a similar crash, while second fastest with ten minutes to go, but was able to get back on and return his Tuenti Derbi to the pits. Espargaro slipped to fifth in the closing stages.
The stormy weather is expected to continue for the rest of the weekend.
Friday practice (2) - Estoril:
1. Moncayo (Aprilia) 2m 5.490s
2. Zarco (Aprilia) +1.063s
3. Terol (Aprilia) +1.297s
4. Rabat (Aprilia) +1.601s
5. Espargaro (Derbi) +1.655s
6. Smith (Aprilia) +1.765s
7. Webb (Aprilia) +1.920s
8. Vazquez (Derbi) +3.169s
9. Cortese (Derbi) +3.204s
10. Savadori (Aprilia) +3.615s
21. Kent (Lambretta) +6.755s