Sullivan remained unbeaten Tuesday by overtaking Duchesne in the second half and winning 64-57.
Sullivan got off to a fast start against Parkway South, but they faded over the final three quarters and suffered their first defeat on Friday.
Sullivan won their fourth and fifth straight games in a 49-25 rout against Warrenton on Monday at SHS, then a rematch against Nerinx Hall on Thursday, 48-45.
Sullivan collected a pair of dual wins Friday at the St. Charles West Quad.
The Eagles handily took care of Priory, 72-12, while edging out the host Warriors, 43-36.
First round results:
Sullivan 56, Northwest 19
Borgia 46, Salem 29
Helias 57, Cuba 27
Owensville 29, St. James 27