It had been 15 years since two undefeated teams met so deep into the WPIAL softball season.
Riverside took advantage of the rare moment to gain a big win Wednesday.
Kirsten Wilson threw a two-hitter with 13 strikeouts and also homered to lead No. 3 Riverside to a 4-0 win against No. 2 Chartiers-Houston in a Class AA semifinal at Fairhaven Park in Kennedy Township.
It was the 11th shutout in a row and 16th of the season for Riverside (18-0). The Panthers have allowed six runs this season.
Chartiers-Houston (18-1) had only one player reach as far as second base.
Wilson's solo home run in the first proved to be the winning run. Cassie Barker and Natalie Pritts also had RBIs.
Chartiers-Houston star pitcher Kayla Briggs was pulled after the second inning after giving up four hits.
The most recent time two undefeated teams met that late in the season was in 1998, when Carmichaels defeated Sto-Rox in the WPIAL Class AA final.
Other Class AA
• Deer Lakes 11, Beaver 7: Defending champion and No. 1 Deer Lakes (18-0) rallied from a 7-0 second-inning deficit to beat No. 13 Beaver (12-7) at North Allegheny. Hannah Blinn and C.C. Sarver each had two-run singles in an eight-run third. Deer Lakes pitcher Tiffany Edwards did not allow a hit after the second. The game was stopped twice because of rain.
• North Allegheny 7, Peters Township 4: Laura Cotter drove in the tying and go-ahead runs with a sixth-inning double and No. 3 North Allegheny (17-4) held on to edge No. 7 Peters Township (18-5) at Fairhaven Park. Bonnie Pierson and Christine Scherer had three hits apiece for North Allegheny, which scored five runs in the sixth.
• The other semifinal between No. 1 Canon-McMillan (19-1) and No. 4 Hempfield (17-5) was postponed until 4 p.m. today at California University.
• Ambridge 5, West Mifflin 4: Hannah Smith hit a three-run home run in the first and Lexi Hilzendeger added a solo home run in the fifth to vault No. 3 Ambridge (16-3) to the win against No. 7 West Mifflin (17-5) at North Allegheny. West Mifflin scored three runs in the seventh. Ambridge pitchers walked eight.
• The semifinal between No. 1 Greensburg Salem (19-2) and No. 12 West Allegheny (12-6) was suspended in the top of the fifth with Greensburg Salem leading, 2-0. Play will resume today at 2 p.m. at California University.
• Carmichaels 9, Burgettstown 0: Lindsey Osborne hit a grand slam and No. 3 Carmichaels (21-2) advanced to the WPIAL final for the second year in a row by defeating No. 2 Burgettstown (16-5) at California University. Erica Burns pitched a two-hitter with eight strikeouts for Carmichaels, which has outscored its three playoff opponents, 34-0.
• Neshannock 9, Frazier 1: Madison Shaffer pitched a three-hitter and struck out 12 in leading No. 1 Neshannock (17-2) to the win against No. 4 Frazier (18-3) at North Allegheny. Neshannock's Rayanna Furst hit a two-run home run and Marissa DeMatteo had three RBIs.