Ahead of the race, the duo expressed little confidence of following up on a front-row lock-out with victory on Sunday.
For the first time in his Formula One career Rosberg clinched back-to-back pole positions after leading away the field three weeks ago in Bahrain.
Rosberg finished a quarter of a second ahead of Hamilton as Mercedes again proved they are the team to beat over one lap this season.
But after Hamilton finished third in China after starting from pole, and Rosberg plummeted to ninth in Bahrain, the team came into this race knowing they had to improve their race pace.
The statistics are at least in Mercedes' favour as 17 times from the 22 races at this track the driver on pole has taken the chequered flag.
And only once in those 22 races has a driver off the front row gone on to clinch victory, that being Michael Schumacher in 1996 when he started from third.
Yet Rosberg and Hamilton were both wary of getting too carried away.
Rosberg said: "I was so surprised we could be that quick today [Saturday], but it all went perfectly in qualifying.
"I'm really happy. It's always a good motivational boost, especially for the team - front row, fantastic!
"But of course I have to be a bit cautious because of what happened in Bahrain, so this is nice, but to be enjoyed with caution."
FOX Sports' LIVE blog coverage of the Spanish GP will start at 6:30pm IST, 8pm HKT
Spanish Grand Prix Qualifying Grid
1. Nico Rosberg (Mercedes) 2. Lewis Hamilton (Mercedes) 3. Sebastian Vettel (Red Bull) 4. Kimi Raikkonen (Lotus) 5. Fernando Alonso (Ferrari) 6. Romain Grosjean (Lotus)
7. Mark Webber (Red Bull) 8. Sergio Perez (McLaren) 9. Felipe Massa (Ferrari) 10. Paul di Resta (Force India) 11. Daniel Ricciardo (Toro Rosso) 12. Jean-Eric Vergne (Toro Rosso)
13. Adrian Sutil (Force India) 14. Jenson Button (McLaren) 15. Nico Hulkenberg (Sauber) 16. Valtteri Bottas (Williams) 17. Pastor Maldonado (Williams) 18. Giedo van der Garde (Caterham)
19. Esteban Gutierrez (Sauber) 20. Jules Bianchi (Marussia) 21. Max Chilton (Marussia) 22. Charles Pic (Caterham)
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A look at the Circuit de Catalunya
Whilst Mercedes have naturally brought a considerable upgrade package for this race, its true depth will not be discovered until Sunday's race.
"To win, it's not good enough to start from pole, you need to have the fastest race car," added Rosberg.
"As we've seen there are so many differences in speeds through the race, then with strategy and everything.
"If you don't have a very, very quick car there's no chance. So, it's just really a need to wait and see.
"But we have tried to understand what went wrong in Bahrain, and we do understand better what went on there and have improved things.
"But I'm not going say I'm very confident I can go for the win. No, the target needs to be a little bit less than that I think - maybe a podium."
As for Hamilton, he also expressed "caution", especially as he surprisingly confirmed being at a loss with his set-up this weekend.
"I haven't had the best of weekends up until now," said Hamilton.
"Obviously, this is still great for us, to be one and two on the grid, but I've just been struggling all weekend generally.
"Even my long runs have been pretty poor. I've just been a little bit lost generally, not really knowing what things to change and which direction to go.
"I've not been 100 per cent comfortable, but I knew the car was good enough to do what we did.
"As for the win, we have to improve our race pace. I hope in the near future we will get our win, but we'll still work for it tomorrow [Sunday]."
Here are some of the stories leading up to the Spanish Grand Prix:
Jenson Button remains hopeful of simply scraping into the points in the Spanish Grand Prix after his worst qualifying performance for 10 months.
Felipe Massa and Esteban Gutierrez have both been given three-place grid penalties for Sunday's Spanish Grand Prix.
Martin Whitmarsh says he will not be quitting as team principal of McLaren despite the team's wretched start to this season.
Nico Rosberg and Lewis Hamilton came first and second at the Spanish Grand Prix qualifying session, ensuring Mercedes earned a third consecutive pole this year.
Pirelli is considering changing the structure of its tyres in the wake of the latest failure this season.