Sainz's performance was all the more impressive considering the Spaniard's car had been wrecked by a loose drain cover that forced the cancellation of first practice after just eight minutes' action.
White smoke and sparks erupted from the Ferrari as the cover punched a hole through the floor, leaving a trail of destruction that triggered an automatic 10 place grid penalty for Saturday night's race.
The survival cell, engine, battery and control electronics were all damaged beyond repair during an incident that Ferrari team principal Frederic Vasseur called "unacceptable."
"I think it's tough for a team to arrive in Vegas and to have this kind of incident," he said as mechanics rebuilt Sainz's car.
Action resumed more than five-and-a-half hours later at 2:30 a.m. following repairs to the 3.8-mile course which runs down the famed Strip and against a backdrop of neon-lit hotels and casinos, but with fans cleared from the grandstands.
Despite the governing FIA recognising "highly unusual external circumstances," Formula One stewards said Sainz would still incur a 10 place grid penalty for Saturday's race.
Alpine driver Esteban Ocon's chassis also needed to be replaced due to damage but he too returned for second practice.
Leclerc's fastest lap of one minute 35.265 seconds was 0.517 faster than Sainz.
Fernando Alonso was third quickest for Aston Martin with Red Bull's Sergio Perez fourth and Valtteri Bottas fifth for Alfa Romeo.
Red Bull's triple world champion Max Verstappen was sixth on the timesheets ahead of a race with little at stake for the Dutch driver, who wrapped up the championship in Qatar last month.
"We managed to do the whole programme which I think was the most important for today," said Verstappen, who has been no fan of all the hype surrounding the event and extended that to the track as well.
"I've had better tracks in my life... there's nothing new that I discovered," he said.
Mercedes seven times world champion Lewis Hamilton was more enthusiastic, however.
"It's incredibly fast and a lot of fun... I'm so glad we got to run again. Not a good FP1 (first practice) but they did a good job to fix it," said the Briton.
MEGA EVENT
Team principals were quick to defend the brand new course and the massive investment that has gone into staging the event for the first time in the heart of bustling Las Vegas, a move that has drawn criticism from local residents.
"I'm still convinced that the event is mega for the F1," Vasseur said, noting that issues with manhole covers had impacted races in Monaco and Baku in the past.
"This is probably going to be one of the most watched Grand Prix in the history of the sport," said McLaren CEO Zak Brown.
"We have a tremendous amount of corporate partners out here, lots of fans, lots of attention on this sport as a whole and I think it's (F1 owner) Liberty taking the sport to yet another level."
Mercedes team principal Toto Wolff lashed out at a reporter who asked how the cancelled practice was not a "black eye" for F1. "That's completely ridiculous," Wolff said.
"How can you even dare to talk bad about an event that sets the new standard, and you're speaking about a ... drain cover that's been undone. It's happened before, it's nothing."
Leclerc was also fastest in the first session, with a time of 1:40.909, followed by the Haas duo of Nico Hulkenberg and Kevin Magnussen. Verstappen was fourth.
The sessions along the transformed Strip marked F1's return to Las Vegas for the first time in nearly 40 years.
(Reporting by Rory Carroll in Los Angeles, additional reporting by Alan Baldwin in London; editing by Miral Fahmy and Toby Davis)
