If there were some cynics in the press room for the triple champion's no-show, it was only because he had joked the previous evening that he might "fake a pulled muscle" to get out of the wet tyre test on an artificially-soaked track.
Trucks had sprayed water around the Circuit de Catalunya since the early hours.
"Electrical fault kept us in the garage this morning, so I've decided with the team not to drive today as I wouldn't have learned much," the Briton said on Twitter.
"Shame not to drive but it's been a great few days. The guys have done an awesome job. Can't wait to be back in the car next week," he added.
Hamilton had made clear on Wednesday night that he was not a big fan of the wet weather tyres.
"I hear it's wet tomorrow, so not particularly looking forward to that," he had told reporters. "I drove some demonstration wet tyres at Silverstone (last week) and they didn't feel great. And I'm out first, unfortunately.
"I might fake a pulled muscle in the morning and let Valtteri do it."
Mercedes had shown impressive reliability up to Thursday, completing far more laps than any other team with barely a hitch.
The two drivers have been sharing the load, with one driving in the morning and the other in the afternoon.
Bottas, who has moved from Williams to replace retired world champion Nico Rosberg, would have been due to start in the afternoon but completed nine laps before the lunch break.
He ended up with 68 to his credit, more than a race distance.
The Finn's fastest lap on Wednesday would have secured pole for any race at the Spanish circuit since it was reconfigured in 2007, and Mercedes have again looked the team to beat with Ferrari closest.
Ferrari's 2007 world champion Kimi Raikkonen ended Thursday with the fastest time and 92 laps covered. France's Romain Grosjean in the Haas did most laps of all, with 118.
Williams did not take part in the final day after Canadian rookie Lance Stroll's crash on Wednesday that damaged the car. Testing resumes next Tuesday at the Spanish circuit.
(Reporting by Alan Baldwin, editing by Pritha Sarkar)
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