Grosjean leapt to the top of the timesheets in the afternoon when he bolted on Pirelli's supersoft compound and dropped down to a 1m 24.067s - the fastest time any driver managed over the four-day test in Spain.
"We made some good improvements with the balance and set-up over the course of the day and it felt a lot better after we made changes for the afternoon," Grosjean said. "We completed a very promising mileage and it's always good to see your name at the top of the timing monitors, even if it is far too early to know what everyone else is doing."
But even before Grosjean hit the front, McLaren had called an early end to their own testing programme following Alonso's crash.
The Spaniard hit the wall on the inside of the track between Turns 3 and 4 shortly before midday. An ambulance was dispatched to the scene, and Alonso was taken to the circuit's medical centre before being airlifted to hospital for further checks.
McLaren subsequently confirmed that Alonso was “conscious and spoke with the doctors”.
Red Bull's Daniil Kvyat had occupied second spot heading into the final hour, but was demoted following a blistering late run from Mercedes' Nico Rosberg.
Despite using Pirelli's medium tyre, Rosberg was able to work down to a 1m 24.321s - putting him within three tenths of a second of Grosjean's benchmark.
Kvyat therefore finished third fastest ahead of Sauber's Felipe Nasr and Williams' Valtteri Bottas, who like Rosberg improved late on.
"It was another good day for us," Kvyat explained. "We learned a lot that will be of benefit in the future and that's what testing is all about. The steps we are making are encouraging and there is plenty of potential there."
Toro Rosso's Carlos Sainz Junior was sixth quickest, although his day ended prematurely when he also crashed exiting Turn 3.
“It was a really good day until I crashed," the Spaniard reflected. "Unfortunately I made a mistake at Turn 3, which is not the best place to do that, because the barriers are close at that point. I'd rather crash in the test than in Race 1, so I have learned something from it and now we turn the page, move on and think about next week's test here.
"There are many positives to take from this test. I was a different driver in the car and I felt much more confident here in Barcelona. Everything went really smoothly and I felt we had made a big step forward with the car from Day 2 to Day 4."
Ferrari's Sebastian Vettel was seventh fastest ahead of Nico Hulkenberg - driving the 2014 Force India VJM07 - and Alonso.
"First of all, I'd like to wish Fernando a quick recovery," Vettel said. "As for today's test, I can say that we have focused mainly on the car's reliability, completing some long-runs, and testing various settings and tyre configurations. We could not run as much as we'd like, but at this stage of winter testing there are always some teething problems you have to deal with. Overall, I can confirm that my feeling with the car is positive."
Alonso managed just 20 laps in total, the fewest of any driver. Rosberg led the way, clocking up 131 for Mercedes.
The teams have just three days before the final pre-season test commences at Barcelona on February 26-March 1.
Unofficial Sunday testing times from Barcelona:
1. Romain Grosjean, Lotus, 1m 24.067s, 111 laps
2. Nico Rosberg, Mercedes, 1m 24.321s, 131 laps
3. Daniil Kvyat, Red Bull, 1m 24.941s, 104 laps
4. Felipe Nasr, Sauber, 1m 24.956s, 73 laps
5. Valtteri Bottas, Williams, 1m 25.345s, 129 laps
6. Carlos Sainz Jr, Toro Rosso, 1m 25.604s, 88 laps
7. Sebastian Vettel, Ferrari, 1m 26.312s, 76 laps
8. Nico Hulkenberg, Force India, 1m 26.591s, 36 laps
9. Fernando Alonso, McLaren, 1m 27.956s, 20 laps