Rachal went to the line and calmly made both free throws.
The victory was Tulsa's first over a top-5 team since winning at UCLA in 1996.
The victory lifted Tulsa to 2-1 in the American Conference and 5-3 overall. Houston (2-1, 7-1) had won its first seven games handily, the closest being a nine-point victory over UCF.
Ahead by one at halftime, Tulsa built a six-point advantage several times in the second half. But the Cougars never lost touch, tying the game at 62-all on an offensive rebound by Marcus Sasser.
Rey Idowu made one of two free throws to give Tulsa a 63-62 lead before Mills put the Cougars ahead.