Perez on top in Bahrain, Red Bull recover
Mexican driver Sergio Perez, of Force India-Mercedes team drives on February 28, 2014 during a four-day testing period at Bahrain's Sakhir circuit ahead of the Bahrain Formula One Grand Prix - by Mohammed al-Shaikh
Perez clocked 1min 35.570sec off a marathon 108 laps with Ferrari's Fernando Alonso just behind on 1:35.634 off 122 laps.
But Red Bull's Australian driver Daniel Ricciardo was third-fastest with a best time of 1:35.743 having completed 66 laps, almost twice as many as Thursday's problem-plagued session.
"It went a lot better today," said Ricciardo, who was a despondent seventh on Thursday.
"In the afternoon we did some good stuff. We had a go on the soft tyre, which was nice, to see how the car responds with a bit more grip.
"We also did one long run at the end, which was very important, just to see how the car is behaving with the tyres. A big improvement everywhere over yesterday and a much more optimistic outlook now I think.
"We still have a lot to learn and understand, but we're making progress and the past two days have been a big step in the right direction."
Ricciardo has now completed his winter testing with world champion Sebastian Vettel to take over the driving duties on Saturday and Sunday before the teams head for Australia and the March 16 Melbourne opener.
Felipe Massa in the Williams was fourth-fastest on Friday with Jenson Button in a McLaren fifth and Marussia's Jules Bianchi a surprise sixth-fastest.
Mercedes endured a difficult day even though Lewis Hamilton managed 89 laps, with a best time of 1:39.041 for seventh spot.
Hamilton had to cut short his afternoon session after the car developed gearbox trouble.
"Like every team, we're encountering problems every day and we've still got some work to do in terms of both reliability and speed," said Hamilton.
"There are no specific problem areas, just lots of different things we need to improve. It's important to get as much mileage under our belts as we can and just as important to discover any problems before we get to Melbourne.
"It might be frustrating at times but finding them at this stage is actually a positive. We really don't know where anyone stands right now.
"Today was a fairly average day but I've got one more to go before we get to Melbourne, so hopefully that will be a bit more productive."
1. Sergio Perez (MEX/Force India) 1:35.570 (108 laps), 2. Fernando Alonso (ESP/Ferrari) 1:35.634 (122), 3. Daniel Ricciardo (AUS/Red Bull) 1:35.743 (66), 4. Felipe Massa (BRA/Williams) 1:36.507 (103), 5. Jenson Button (GBR/McLaren) 1:36.901 (52), 6. Jules Bianchi (FRA/Marussia) 1:38.092 (75), 7. Lewis Hamilton (GBR/Mercedes) 1:39.041 (89), 8. Jean-Eric Vergne (FRA/Toro Rosso) 1:39.636 (61), 9. Esteban Gutierrez (MEX/Sauber) 1:39.976 (106), 10. Pastor Maldonado (VEN/Lotus) 1:41.613 (31), 11. Marcus Ericsson (SWE/Caterham) 1:42.516 (55)