The 25-year-old Lahiri, a two-time Asian Tour winner, ensured himself a memorable maiden Open appearance with a battling two-over-par 72 which placed him on level par 140 at Royal Lytham and St Annes.
American Brendt Snedeker holds the halfway lead on 130 but Lahiri is only a couple of shots outside the top-10.
Singh, winner of the Scottish Open last week, fired a 71 to lie on 141 and along the way celebrated his own milestone by completing his streak of making the cut in all the four Majors.
"Excellent," Singh said in a media release issued by Asian Tour, when told that Lahiri was also through to the weekend. "First British Open for him and it's great that he's done well. First time we have two Indians in the weekend rounds. It speaks well that the Indians are doing well."
"I'm very proud I've made the cut at the Open. This is the only Major that I've not made the cut. This is another feather in the cap. This one was missing and I've done that too. It's a cut slam!"
Singh, a two-time Asian Tour number one, is not discounting a weekend charge after his play-off triumph in Scotland. "I've got confidence in my game. You never know on links course. If the wind picks up, anything can happen. It'll give us a chance to make a run," said Singh.
India's rising star Lahiri was delighted to accompany Singh into the history books for Indian golf.
"I'm happy. I'm really happy that I'm playing in the weekend. If someone had said even par after two rounds, I would have taken it on Thursday morning. I hung in there. I had to make a few crucial putts and I stuck to my routine. I told myself to go out there and enjoy myself," said the 25-year-old.
"It's been very special. My dad has been walking with me and my manager is here. I can't ask for anything more. I'm happy I put in a decent performance for them."
After shooting only one of three bogey-free rounds on day one en route to a 68 on Thursday, Lahiri said he did not know what the leading score was as he wanted to focus solely on his mission.
"When we got home last night, dad opened up his laptop but I told him not to tell me what the scores were as I just wanted to keep doing my thing," said Lahiri, who practices meditation to help him concentrate.
"I told him you can read all you like but I want to stick to my game plan. I don't want to let what others are doing affect me, at least not until Saturday afternoon. I like to be in my own comfort zone, to be in my routine to do what I feel will help me play best. I've been doing that and it was fun."
Lahiri was especially proud with the way he handled the pressure, especially after he turned in two over with bogeys on five and nine. "I didn't hit it as well as I would have liked," he said.
"I wasn't thinking about the cut line. After I made bogey on nine, I was really upset. I made some bad clubbing decision and I seemed to lose my concentration after that. On the par five 11, I hit it to the singular worse spot that you could. But I made a good 10 footer for par and that saved me from losing it."
"I was on the verge of getting angry and I didn't get angry. That was very critical as on a course like this, if you lose your temperament you can lose it. I'm glad I managed to keep myself calm."