Lahiri, who is ranked 217 in the world, carded five-under 66, the same as England's Robert Coles and Welsh player Rhys Davies.
Lahiri admitted he was lucky to start in the morning and he made good use of the weather conditions, which grew worse as the day progressed.
"The conditions were good early morning when we started but it just kept getting worse and worse," said Lahiri, who this year made the cut at The Open at Royal Lytham and finished tied 31st.
A good number of Indians are in the field.
Gaganjeet Bhullar, a teammate of Lahiri, when India won an Asian Games silver medal in 2006, shot 72 and Chiragh Kumar, runner-up at the 2011 Indian Open, carded 77.
Other Indians, who were still on course or were yet to start included SSP Chowrasia, Jyoti Randhawa, Shiv Kapur and Digvijay Singh.
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Despite the cold and heavy rains at the Crans-sur-Sierre Golf Club, Lahiri, a two-time Asian Tour winner, showed that he could play in such adverse conditions.
"This is definitely the best round that I had since I started playing here. I've come into this event not in the best of form and state of mind before but this year it has been a lot different. I'm very focused and confident," said Lahiri, who has credited his Vipassna meditation for turning his performances around.
"It's scheduled for more rain tomorrow morning and I think I got lucky with the draw that I'm playing in the afternoon tomorrow, so we'll see how it goes." (MORE)