Tiago Monteiro of Mexico was fastest in Saturday's qualifying session at Autodromo Hermanos Rodriguez with a speed of 112.639 miles per hour, moving from ninth to second on the grid.
But Tracy, who leads the series with six wins, had his speed of 112.893 mph stand from Friday's session. He drove his Ford/Lola around the 2.786-mile road course in 1 minute, 28.842 seconds.
With three races remaining, Tracy has 205 points, 14 more than Brazil's Bruno Junqueira, who will start third Sunday.
Junqueira was just .005 seconds slower than Tracy on Friday, clocking 112.813 mph in a Ford/Lola. The Brazilian finished third here last year.