Muruguza trounced top-ranked defending champion Karolina Pliskova 6-3, 6-2. With the win, the 23-year-old snapped a six-match losing streak against the current world No. 1, and reached her first final since Wimbledon last month.
On the other hand, Halep, a finalist in 2015 and a semifinalist here last year, comfortably dismissedSloane Stephens 6-2 6-1 in just 54 minutes.
The win has now given her a shot at becoming the first Romanian champion in Cincinnati since Magda Rurac won in 1949.
Dimitrov has won his only previous meeting against Kyrgios, taking out the 22-year-old in a third-set tiebreak at Indian Wells in 2015.
Kyrgios, currently ranked No. 23 in the world, pulled out of three straight tournaments this summer due to hip and shoulder injuries, finally notching two wins last week in Toronto before falling in the round of 16.