Half the field in the 4th U.S. Women’s Amateur Four-Ball Championship has been officially trimmed. Only 32 sides (64 players) remain for the match-play portion of the competition at El Caballero Country Club. The average age of those 64 players is 18.1 and only four are mid-amateurs (25 and older).

This is what soccer followers call the knock-out rounds. It’s now survive and advance.

By the end of play on Monday, only 16 sides will be left and we’ll be one step closer to crowning the USGA’s first national champions of 2018.

Here are five matches to keep an eye on (all times PDT):

Katrina Prendergast/Ellen Secor vs. Calynne and Chandler Rosholt (9:06 a.m.)

The two Colorado State standouts face one of the two remaining sister tandems. Prendergast, of Sparks, Nev., and Secor, of Portland, Ore., are no strangers to this championship, having advanced to the Round of 16 a year ago at The Dunes Golf & Beach Club. Prendergast, a junior, registered a win and two other top-five finishes this past season for the Rams. Secor, a sophomore who was diagnosed with Type 1 diabetes at age 3, is competing in her third Women’s Amateur Four-Ball, having qualified for the 2016 event with Jessica Sloot. This is Calynne Rosholt’s first USGA championship, while big-sister Chandler lost in the Round of 32 last year with partner Macy Fox. She is headed to Auburn University this fall.

Haley Moore/Gigi Stoll vs. Abbey Daniel/Ashley Gilliam (9:18 a.m.)

Moore, of Escondido, Calif., and Stoll, of Tigard, Ore., came into this championship from the Pacific-12 Conference Tournament in Seattle, Wash., where they helped the University of Arizona finish third. Moore also finished third individually. Daniel, of Covington, La., and Gilliam, of Manchester, Tenn., are each past national finalists in the Drive, Chip & Putt Championship. Both have verbally committed to attend Mississippi State. Gilliam helped Tennessee claim the final USGA Women’s State Championship title last fall.

Momoka Kobari/Hira Naveed vs. Melena Barrientos/Avery Zweig (9:54 a.m.)

The two Pepperdine University standouts face the youngest team in the field with a combined age of 25. Kobori, a sophomore who was named the West Coast Conference’s Freshman of the Year in 2017, posted a pair of top-10 finishes this season, while the Australian-born Naveed, a junior, tied for second in the recent West Coast Conference Championship. Zweig, 11, is a two-time national Drive, Chip & Putt finalist who is the youngest competitor to ever reach match play in the championship’s four-year history. She and Barrientos reside in suburban Dallas and met each other through junior golf.

Ty Akabane/Briana Chacon vs. Smith Knaffle/Skylar Thompson (10:54 a.m.)

This is a future Pac-12/Southeastern Conference showdown with Californians Akabane and Chacon both committed to play at the University of Oregon, while Knaffle, of Murrells Inlet, S.C., and Thompson, of Buford, Ga., are future South Carolina Gamecocks. This is Akabane’s sixth USGA championship, including a start in the 2017 U.S. Women’s Open. Thompson, a lefty, is a past national Drive, Chip & Putt finalist.

Yachun Chang/Lei Ye vs. Whitney Britton/Avery French (11:18 a.m.)

Chang, of Chinese Taipei, and Ye, of the People’s Republic of China, have proven they can go low with their first-round 64, which matched the 18-hole championship scoring record. Chang, a quarterfinalist in the 2016 U.S. Girls’ Junior, is headed to the University of Arizona this fall, while Ye has committed to attend Stanford University in 2019. Britton, of Dana Point, Calif., and French, of Laguna Niguel, Calif., are one of two sister tandems to qualify for match play. Britton, 27, advanced to the semifinals of the 2015 U.S. Women’s Mid-Amateur and is one of four mid-amateur golfers to qualify for match play this week. Last year, the tandem lost in the Round of 32.

David Shefter is a senior staff writer for the USGA. Email him at dshefter@usga.org.