Red Bull's frustrating start to Formula One testing came to a sputtering halt on Friday as four days of car problems ended with another early retirement, this time due to a mechanical fault.
Daniel Ricciardo managed just seven laps around Jerez Circuit before pulling out of the morning session to focus on getting the RB10 ready for the next preseason test in Bahrain.
Williams led the time sheets with Felipe Massa clocking a best lap of 1 minute, 28.229 seconds from 89 laps, nearly 1 second ahead of former Ferrari teammate Fernando Alonso. Alonso ran a session-high 115 laps, with McLaren driver Kevin Magnussen following with 110 laps.
Red Bull said "a mechanical issue" was discovered that could not be fixed in time for the afternoon session, leaving it to run just the seven laps.
In all, Red Bull managed just 21 laps over four days due a variety of issues, from poorly-fitted parts to battery problems and electrical issues.
Four-time defending champion Sebastian Vettel managed just 14 laps, while championship rivals like McLaren and Ferrari ran strong test programs ahead of the new season.
"It's been a very difficult test," Red Bull team principal Christian Horner said. "We have had numerous Renault issues as well as chassis cooling issues, which have affected our progress. However, despite the lack of mileage, what we have managed to learn shows that the problems should be solvable for the next test in Bahrain."
Mercedes pair Nico Rosberg and Lewis Hamilton combined for a promising 132 laps on Friday, as teams grappled with a rash of regulation changes that have affected everything from weight and aesthetics to the power of the engine.
"I was able to do a full race simulation on only the fourth day of winter testing, so we can be very happy about that," said Rosberg, who lapped a quickest time of 1:36.951 to trail Hamilton by six seconds. "We've learned a lot this week so it's an encouraging start to the year."
Mercedes led the test with 309 laps to Ferrari's 251. Lotus did not attend the opening pre-season test.
Early morning rain also allowed teams to test in the wet, with the day finally ending under a red flag after Magnussen ran into the barriers, with Daniil Kvyat of Toro Rosso also finding the same wall.
The second preseason test at Bahrain runs from Feb. 19-22.