The session was run under sunny skies with chilly 43-degree temperatures.
Westbrook turned a lap of 1:39.462 in the No. 90 Spirit of Daytona Racing Corvette DP.
Christian Fittipaldi, driving the No. 5 Action Express Racing Corvette DP, was second, 1:39.764, followed by defending winner Scott Pruett, running 1:40.147 in the No. 01 Telcel/Target Ford EcoBoost/Riley.
Antonio Garcia, overall winner of the 2009 Rolex 24, was fastest in GTLM in the Corvette Racing No. 3 Corvette C7.R, 1:46.036, followed closely by Marc Goossens in the No. 91 SRT Motorsports SRT Viper GTS-R, and Darren Turner, 1:46.101 in the No. 97 Aston Martin Vantage.
Oliver Jarvis led GTD, running 1:47.892 in the No. 46 Fall-Line Motorsports Audi R8 LMS. All but one of the 67 cars entered in the Rolex 24 participated in the session, with the No. 555 Level 5 Motorsports Ferrari 458 Italia failing to turn a lap.
The one-hour session was red-flagged twice. Alex Riberas was unhurt in an incident in Turn 5 in the No. 23 Team Seattle/Alex Job Racing Porsche 911 GT America. The other stoppage came when James Davison came to a stop in speedway Turn 4 in the No. 007 TRG-AMR Aston Martin Vantage.
The second practice session will run from 1:15-1:45 p.m. ET, followed by qualifying starting at 3:45 p.m.
There will also be a 90-minute night practice session beginning at 6:30 p.m. - See more at: http://www.imsa.com/articles/westbrook-leads-opening-rolex-24-practice#sthash.UNJ42gqJ.dpuf