"I think this is more or less normal in every new auto", Alonso said of the problems faced.
The two-time F1 world champion completed 46 laps before an oil leak forced him to stop on track at Turn 7 two hours into the day's track action.
Ferrari had a productive day as Sebastian Vettel took over in the morning, before handing the duties to teammate Kimi Raikkonen later in the day. Mercedes clocked 177 laps between Hamilton and Bottas, while Vettel racked up 171 on his own for Ferrari.
"There are three quick teams and there's no doubt that Red Bull are going to be one of the teams we're going to be fighting with this year", Mercedes technical director James Allison told Sky Sports after seeing Ricciardo go fastest.
The reigning champion made gradual progress up the order throughout the afternoon, eventually overhauling Pierre Gasly's Toro Rosso using soft tyres and then improving to a best time of 1m20.808s. "The issues we have, they are well under control. Of course, you would like to do more laps but we don't think we have any problems so we are not concerned".
As with the rest of the field, McLaren spend nearly the entire day in the garage after snow falls in Barcelona.
After an early problem in the pitlane, the MCL33 breaks down on track due to a battery problem. Today, it was an oil leak, which kept Fernando Alonso in the garage rather than at the track for almost seven hours. Alonso only returns for final 13 minutes.
Ricciardo's effort usurped Felipe Massa's time from 2008 as the fastest lap of this configuration of the Spanish Grand Prix circuit. McLaren now has the lowest lap count over the first six days after an oil leak limited Alonso to 57 laps on Wednesday, but the Spaniard says the problems that have hit so far are all minor and get highlighted by the media presence at the tests. In addition, the lap was a second quicker than Hamilton's pole time for the Spanish Grand Prix past year.
Esteban Ocon also passed a century of laps, although he was only 12th fastest in the Force India.
Haas struggled with an oil leak but were able to fix the issue, allowing Romain Grosjean to put in some late laps.