Earl Bamber led the way for Porsche as the Mexico round of the World Endurance Championship hit the three-hour mark.
Driving the championship-leading 919 Hybrid shared with Brendon Hartley and Timo Bernhard, Bamber held a 20.324 second lead over Andre Lotterer in the sister Porsche shortly after the third round of pit stops.
Hartley consolidated his pole position with a double stint before handing over to the New Zealander just before the two hour mark.
At times during the first half the two Porsches were less than a second apart, but the margin increased when the #1 car picked up a drive through penalty for speeding in the pit lane.
Toyota's TS050s ran a distant third and fourth at halfway, with the #8 car of Kazuki Nakajima running ahead of the #7 driven by Jose Maria Lopez.
In LMP2, Vaillante Rebellion led G-Drive Racing by 27 seconds following an impressive opening stint by Bruno Senna.
The fast-starting Brazilian moved into the lead from third on the grid before working into a comfortable advantage, before handing the #31 ORECA-Gibson over to team-mate Nicolas Prost.
Prost then consolidated the advantage, while the G-Drive ORECA and Signatech Alpine A470 continue to fight for second behind.
An eventful opening half in GTE-Pro culminated with Ferrari's Sam Bird leading Nicki Thiim in the best-placed Aston Martin by seven seconds.
Bird lost his pole position to Thiim's co-driver Marco Sorensen at the start of the second hour, before the Ferrari moved back ahead during the first round of stops.
So far, GTE-Pro has been the most eventful category, with each manufacturer having one car involved in incidents.
Aston Martin's #97 Vantage dropped out of contention early on with a braking issue, shortly before the #92 Porsche lost ground when Michael Christensen spun following contact with a GTE-Am runner.
The most notable incident of the first three hours was a collision between the #66 Ford of Olivier Pla and #51 Ferrari of James Calado, which forced both to make emergency repairs.
The pair touched whilst battling through the stadium section during the third hour, leaving the Ferrari with a left-rear puncture and the Ford with damage to its right-rear side.
Following a quick service, the Ferrari managed to continue in third, ahead of the #91 Porsche, while the Ford dropped to seventh in class.
GTE-Am was headed by the Proton Competition Porsche 911 RSR, which lost its lead at the start before Matteo Cairoli restored the original order with an overtake on Mathias Lauda's Aston Martin midway through hour three.
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