What happened: Isack Hadjar slowed on track after reporting an issue with his Red Bull during opening practice for the Mexico City Grand Prix. The French driver guided the car back to the pit lane at reduced speed as mechanics prepared to examine the issue.

Why it matters: The stoppage costs Red Bull valuable setup data at high altitude ahead of Sunday's race. Hadjar returned to Red Bull in Baku earlier this season following a wrist injury.

What to watch: Red Bull will work to diagnose and repair the issue before second practice later Friday.