The Brit arrived in Sochi seven points adrift of Vettel after the Ferrari driver claimed victory number two of the new season at the last race in Bahrain.
The Mercedes driver has won two of the three races staged in Russia and will be desperate to make amends following his defeat at the Sakhir Circuit to Vettel a fortnight ago.
But it was the German, and his resurgent Ferrari team, who appeared on course to strengthen their grip on the new championship after a dominant display at the Sochi Autodrom.
First, his Ferrari team-mate Kimi Raikkonen posted the fastest lap in opening practice before Vettel soared to the top of the timesheets during the day's second running.
Vettel set a best lap of one minute and 34.120 seconds at this 3.1-mile circuit which runs through the Olympic Park - the site of the 2014 Winter Games - to finish ahead of Raikkonen, while Hamilton was only fourth.
Hamilton was slower, not only to both the Ferrari drivers, but his Mercedes team-mate Valtteri Bottas too as he ended the 90-minute session a distant 0.709 seconds adrift of Vettel's best time.
There is plenty of work to do then for the Briton should he wish to return to the winners' circle on Sunday and get his championship assault back on track.
"The Ferraris are definitely the quickest as they have been all year so far," Hamilton said. "We will work hard this evening to see if we can pick up some time. Their pace does not surprise me because we are struggling getting these tyres up to temperature.
"We are not the quickest, and of course I want to be, but rather than get concerned about it I just put my energy towards trying to figure out how we can improve."
Vettel suggested that Hamilton's Mercedes team were "sandbagging" on Friday in a deliberate ploy not to give away their true pace. But Hamilton scoffed at his rival's claim.
"I don't know why they would suggest that," Hamilton added. "That is just Sebastian trying to blow smoke our way. Those guys are very quick.
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"If we get our tyres working we can get closer and the gap is still very, very close, but we are just not as quick as we need to be."
Behind Hamilton, Dutch teenager Max Verstappen was fifth in the order, but he missed the final 20 minutes of the session after he stopped on track with a mechanical failure.
"I lost power," he said.
His team-mate Daniel Ricciardo finished sixth, but was an eye-watering 1.8 seconds slower than Vettel as Red Bull showed few signs of reducing the deficit to Ferrari and Mercedes.
Double world champion Fernando Alonso arrived in Russia directly from America after a recent seat fitting ahead of his upcoming soiree to the Indianapolis 500.
And the Spaniard, who is yet to finish a race this season following McLaren and their engine supplier Honda's troubles, was 12th in the order with his team-mate Stoffel Vandoorne, who will serve a 15-place grid penalty for already exceeding the number of engine parts allowed for the new season, four places further back.
British driver Jolyon Palmer is in need of an improved weekend following a difficult start to his second campaign in the sport. The 26-year-old from Horsham was 13th, four tenths of a second and five spots behind his Renault team-mate Nico Hulkenberg in the timesheets.