"We want to get past [the Pandas] and get to the first round," said Maddox, whose team is 8-15, but finished third in the Orange Coast League to make its third straight trip to the playoffs.
Immaculate Heart (13-10) was 3-5 in the Sunshine League, but its overall record helped it receive one of the division's six at-large bids. It is not known when the Pandas last reached the postseason, but they have not competed in the playoffs for at least the six preceding seasons.
Coach Claire Rietmann-Grout has just two seniors.
Costa Mesa is anchored by seniors, including shortstop and captain Alex Kronfeldt, second baseman Sami Feinstein and pitcher Tamy Cerrato.
Kronfeldt has scored 21 runs and has two home runs and six runs batted in.
Feinstein has scored 21 runs, has nine stolen bases and seven of her eight hits are for extra-bases, including five doubles.
Cerrato has a 10.30 earned-run average, but has been the primary starter all season.
Sophomore Molly Settles is another standout for the Mustangs. She is batting .385 with 20 hits.
Senior catcher Alosina Skipps is another stalwart, Maddox said.
The Mustangs have come close in the postseason. After breaking a five-year drought that followed a trip to the playoffs in 2003. In 2009, Mesa fell, 3-2, in its opening playoff game to Western.
Immaculate Heart is paced by sophomore pitcher Jessica Osnaya, who is 7-6 with a 2.28 ERA and has 92 strikeouts in 83 innings.
Senior Maeve Brophy is batting .375 with 12 hits, nine RBIs and three home runs, while sophomore Kaylan Rothrock is batting .339 with 21 hits, 11 RBIs and 15 runs for the Pandas.
Tuesday's winner is at Mission Valley League champion Arroyo (16-6) in Thursday's first round.