Hulkenberg, also fastest in the morning, shaved around two seconds off his lunchtime benchmark using the supersoft tyres to work his way down to a 1m 23.110s - the second fastest time seen so far in testing, and comfortably quick enough for the German to end the day on top.
“A positive day for me and I’ve come away with some good first impressions as we continue to learn about this car," said Hulkenberg. "We tried quite a lot of things today and, although we need to do some more fine-tuning, it does feel like we are moving in the right direction."
Grosjean meanwhile continued Haas’s impressive start to their inaugural season by vaulting into second late on - albeit nearly three seconds down on Hulkenberg.
Kimi Raikkonen was third fastest in what was a disjointed first day in Ferrari’s new SF16-H.
The Finn only set four laps - none timed - in the morning due to fuel system checks, but enjoyed a more productive afternoon, clocking up close to 80 laps and finishing third on the timesheets, fractionally ahead of the Renault of Kevin Magnussen.
"These are early days, especially for me with this car," said Raikkonen. "We are happy so far, even if we know that there are still a lot of things to do.
"It was painful losing time today, but we always try to do our best. There are always mistakes but we always want to improve and learn from them."
Magnussen suffered his own technical glitch late on, but was able to return and was one of four drivers to break into triple figures in the lap count - a welcome day after separate issues halted the team on the first two days.
“It’s fantastic to get back in a Formula 1 car after eleven months away," the Dane commented. "Everything felt good straight away and it was great to be working with the team for the first time behind the wheel. We completed a lot of laps which is a real positive and I can’t wait to jump in the car tomorrow and continue today’s work.”
Mercedes opted to split the day between their two drivers in order to keep them fresh after mammoth stints on the first two days - and the trend continued as Nico Rosberg and Lewis Hamilton surpassed 150 laps combined.
The German was the faster, booking fifth for Mercedes, while Hamilton - driving in the afternoon - was half a second down in eighth.
Splitting the pair was the Toro Rosso of Carlos Sainz and the Sauber of Felipe Nasr - with both men also enjoying huge mileage.
Sainz was the busiest of all drivers, moving past the 160-lap mark on his second day in the new Ferrari-powered STR11, while Nasr managed 115 in the 2015-spec Sauber.
Red Bull’s Daniil Kvyat was ninth, narrowly ahead of Williams’ Felipe Massa and McLaren’s Jenson Button. The Briton managed just 51 laps, the fewest of any driver, and had his day brought to an early end due to a hydraulic leak.
Rio Haryanto, making his first official appearance as a Formula One driver, completed the timesheet in 12th. The Indonesian brought the red flags out when he spun at Turn 5 in the morning, and ended up more than 5s off the pace - although he also elected to abandon a faster lap in the closing stages of the day.
Wednesday unofficial testing times from Barcelona
|1||Nico Hulkenberg||Force India-Mercedes VJM09||1:23.110||99|
|2||Romain Grosjean||Haas-Ferrari VF-16||1:25.874||2.764||82|
|3||Kimi Raikkonen||Ferrari SF16-H||1:25.977||2.867||78|
|4||Kevin Magnussen||Renault R.S.16||1:26.014||2.904||111|
|5||Nico Rosberg||Mercedes F1 W07 Hybrid||1:26.084||2.974||74|
|6||Carlos Sainz||Toro Rosso-Ferrari STR11||1:26.239||3.129||161|
|7||Felipe Nasr||Sauber-Ferrari C34||1:26.392||3.282||115|
|8||Lewis Hamilton||Mercedes F1 W07 Hybrid||1:26.421||3.311||88|
|9||Daniil Kvyat||Red Bull Racing-TAG Heuer RB12||1:26.497||3.387||74|
|10||Felipe Massa||Williams-Mercedes FW38||1:26.712||3.602||109|
|11||Jenson Button||McLaren-Honda MP4-31||1:26.919||3.809||51|
|12||Rio Haryanto||Manor Racing-Mercedes MRT05||1:28.249||5.139||78|