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Lewis hit nine birdies and no bogeys on the par-72 course to lead by two shots from Dutchman Joost Luiten, with France's Alexander Levy and English trio Graeme Storm, Simon Dyson and Richard McEvoy a shot further back.
Lewis won the Portugal Masters on only his third professional start in 2011, but has not had a top-10 finish in a strokeplay event since.
"It's a great start," said the former amateur star, who came into the event on the back of six consecutive missed cuts.
"I had some good fortune around the 17th and 18th, which was my front nine, and that made the back nine more relaxed.
"I holed a lot of putts, which was really the only difference between today and the rest of the year.
"This year's been tough but I've been very close and shown a lot of good stuff. Today it all came together and if I can have three more rounds like that I'll be alright."
Already the oldest winner in European Tour history, Spain's Miguel Angel Jimenez set about breaking his own record with a five-under-par 67, while defending champion Bernd Wiesberger had consecutive birdies at the sixth and seventh as he opened with a four-under 68.
Leading first round scores in the European Tour Lyoness Open at Atzenbrugg (GBR and IRL unless stated, par 72):
63 - Tom Lewis
65 - Joost Luiten (NED)
66 - Richard McEvoy, Simon Dyson, Alexander Levy (FRA), Graeme Storm
67 - Andy Sullivan, Paul Waring, Miguel Angel Jimenez (ESP), Matthias Schwab (AUT), Wen-Chong Liang (CHN)
68 - Jason Barnes, Steve Webster, Shiv Kapur (IND), Daniel Brooks, Luke Goddard, Callum Macaulay, Romain Wattel (FRA), Bernd Wiesberger (AUT)
69 - Damien McGrane, Magnus A Carlsson (SWE), Mark Tullo (CHI), Matthew Nixon, Andrew Dodt (AUS), Chris Paisley, Moritz Lampert (GER), Joachim B Hansen (DEN), Eduardo De La Riva (ESP), Ignacio Garrido (ESP), Sam Little
70 - Craig Lee, Jacob Glennemo (SWE), Ake Nilsson (RSA), Lasse Jensen (DEN), Daniel Gaunt (AUS), Soren Hansen (DEN), Jorge Campillo (ESP), Peter Hedblom (SWE), James Morrison, Anthony Snobeck (FRA), Jamie Elson, Alastair Forsyth, Dodge Kemmer (USA), Gregory Bourdy (FRA)
71 - Soren Kjeldsen (DEN), Mikael Lundberg (SWE), Lukas Nemecz (AUT), Maximilian Kieffer (GER), Morten Orum Madsen (DEN), Rene Gruber (AUT), Thomas Bjorn (DEN), David Higgins, Bradley Dredge, Hennie Otto (RSA), Bjorn Akesson (SWE), Scott Arnold (AUS), Peter Gustafsson (SWE), Phillip Price, Matthew Baldwin, Terry Pilkadaris (AUS), Oscar Floren (SWE), Lee Slattery, David Drysdale, Jurgen Maurer (AUT), Leonhard Astl (AUT)
