Nico Rosberg made up for lost time when he put in a marathon 148 laps at the Jerez circuit on Thursday.
Mercedes lost almost two full days of running when an electrical problem sidelined Rosberg on Tuesday before a brake pressure issue put his team-mate Lewis Hamilton into the barriers on Wednesday.
However, the gremlins appeared to be behind them on Day Three as Rosberg amassed 148 laps as he worked on aerodynamic evaluations, including Merc's new front win design.
"I'm very happy with today and I think it was probably a personal record for me to do almost 150 laps," said the German.
"I've never been so well prepared physically as I am this season; I feel on top form and it was no problem to complete the programme.
"To drive the distance from Silverstone to Spa in one day with no problems whatsoever is a fantastic achievement from the team. It really made up for the last two days and put a smile on everybody's face."
But the W04 was not just reliable on Thursday, it was also quick as Rosberg finished with the day's second best time, a 1:8.766.
"In terms of driving the car, I have a good feeling, I can push it and feel comfortable - but obviously we still have plenty of work to do to continue improving," he added.
Rosberg will hand the car back to Lewis Hamilton for Friday's final day of testing at the Spanish track.