Pacific, seeded 12th, upset fifth-ranked Providence with a 66-58 win in the NCAA 'March Madness' college basketball tournament on Friday.
It was the second upset by 12th seeds over fifth - Manhattan beat Florida 75-60 in the East Rutherford Regional on Thursday.
Pacific's win, with Miah Davis scoring 19 points, came in the St Louis Regional, where number nine UAB edged eighth-seeded Washington 102-100.
The Tigers, Big West champions, had not won a first round game in the NCAA Tournament since 1967 and they will now meet number four Kansas, easy 78-53 winners Illinois-Chicago (13).
Other games in the St Louis - and in the other regionals - went according to plan.
Top seeds Kentucky had few problems with 16th-ranked Florida A&M, winning 96-76.
Third seeds Georgia Tech struggled but still got past 14th seeds Northern Iowa 65-60, while Boston College (6) were 58-51 winners over Utah (11).
In the East Rutherford, Pittsburgh (3) survived a shaky start to rally past 14th-ranked UCF 53-44.
Memphis (7) stopped 10th seed South Carolina 59-43, Oklahoma State (2) beat 15th-ranked Eastern Washington 75-56, and Wisconsin (6) were 76-64 winners over Richmond (11).
In the only games in the Phoenix Region, third seeds North Carolina State beat LA Lafayette 61-52 and Vanderbilt (6) triumphed 71-58 over Western Michigan (11).
Atlanta Region scores:
Illinois (5) 72-53 Murray State (12)
Cincinnati (4) 80-77 East Tennessee State (13)
Mississippi State (2) 85-52 Monmouth (15)
Xavier (7) 80-70 Louisville (10)