Carlos Sainz Jr set the pace for a second day in a row during official GP3 Series testing at Estoril in Thursday.
The 18-year-old Spaniard, who is switching to the championship from Formula 3 this year, leapt to the head of the timesheets in the final 30 minutes of running.
The MW Arden driver's 1m27.654s lap was enough to knock Manor driver Tio Ellinas off the top spot by 0.318s, Trident's Giovanni Venturini then splitting the pair even later in the day with a lap 0.285s slower than Sainz.
Conor Daly was the best of the ART drivers in fourth, ahead of Sainz's team-mate Daniil Kvyat and Kevin Korjus of series newcomer Koiranen Motorsport.
The day's running began in the wet after heavy morning rain, and this limited many drivers' running early on.
By lunchtime a dry line had emerged, however, allowing David Fumanelli - eighth overall - to set the pace ahead of the hour-long break.
The tricky conditions caught out several drivers with Jimmy Eriksson, Patrick Kujala, Facu Regalia and Ethan Ringel all bringing out the red flags due to spins. Samin Gomez did likewise during the afternoon.
New team Bamboo Engineering gave Irishman Ryan Cullen a run-out in the afternoon in place of Lewis Williamson - fourth in the morning - while Melville McKee took over Adderly Fong's Status car for the afternoon.
Pos Driver Team Time Laps
1. Carlos Sainz Jr MW Arden 1m27.654s 31
2. Giovanni Venturini Trident 1m27.939s 26
3. Tio Ellinas Manor 1m27.971s 40
4. Conor Daly ART 1m28.026s 32
5. Daniil Kvyat MW Arden 1m28.071s 34
6. Kevin Korjus Koiranen 1m28.151s 29
7. Alex Fontana Jenzer 1m28.156s 26
8. David Fumanelli Trident 1m28.261s 27
9. Jack Harvey ART 1m28.517s 22
10. Facu Regalia ART 1m28.722s 28
11. Patrick Kujala Koiranen 1m28.729s 27
12. Nick Yelloly Carlin 1m28.734s 29
13. Emanuele Zonzini Trident 1m28.787s 37
14. Dino Zamparelli Manor 1m29.045s 32
15. Aaro Vainio ART 1m29.191s 26
16. Robert Visoiu MW Arden 1m29.342s 22
17. Samin Gomez Jenzer 1m29.556s 35
18. Roberto La Rocca Bamboo 1m29.570s 26
19. Jimmy Eriksson Status 1m29.574s 34
20. Josh Webster Status 1m29.640s 34
21. Ethan Ringel Bamboo 1m29.748s 22
22. Eric Lichtenstein Carlin 1m29.768s 40
23. Patric Niederhauser Jenzer 1m29.808s 32
24. Melville McKee Status 1m30.241s 43
25. Vicky Piria Manor 1m30.383s 33
26. Luis Sa Silva Carlin 1m30.425s 33
27. Ryan Cullen Bamboo 1m31.347s 33