Freshman A.J. Wright had an RBI double for eighth-seeded Cherry Hill West (15-10).
"They outplayed us," Cherry Hill West coach Dan McMaster said of Williamstown. "They swung the bats, and they made every play in the field. They were the better team today."
Rizzo, who is among South Jersey's leaders with seven home runs, said he worked on driving inside pitches during batting practice on Sunday.
"Lucky me. I was looking for inside pitches, and I saw a couple of them," Rizzo said. "I saw them pretty good and hit them pretty far."
Rizzo was 3 for 3 with five RBIs. He had an RBI single in the first inning, when Williamstown broke to a 4-0 lead against Cherry Hill West ace Brenden Del Monte.
"When you have a tremendous pitcher, at any level of baseball, if you're going to get to him you've got to get to him early," Williamstown coach Jim Ambrosius said. "When you jump out like that, it helps everybody loosen up and settle in a little bit."
Mosley was warming up in the bullpen as his team took a 4-0 lead.
"That was a big boost to my confidence," Mosley said.
Mosley, who improved his record to 4-2, said he had his "best stuff" of the season. He struck out five, walked two and hit two. He faced just 27 batters.
"He had his 'A' stuff," Ambrosius said of Mosley, who plans to pitch next year for Gloucester County College. "His breaking ball had a late bite."
Mike Pollastrelli's RBI triple, Rizzo's RBI single and John Brady's two-run double were the big hits for Williamstown in the first inning. Rizzo added a solo home run in the third and a three-run home run in the seventh.
Rizzo's first home run cleared the left-field fence, which is marked with a 325-foot sign, with plenty of room to spare. His second home run went even farther and probably traveled close to 400 feet.
"He's a great guy to have in the middle of the lineup," Ambrosius said. "I think he's one of the best hitters in South Jersey."
Williamstown 401 100 3 – 9 8 2
Cherry Hill West 001 000 0 – 1 2 0
WP: Mike Mosley. LP: Brenden Del Monte. 2B: W-John Brady 2, Anthony Racobaldo; CHW-A.J. Wright. 3B: W-Mike Pollastrelli. HR: W-Jordan Rizzo, 2.
Contact staff writer Phil Anastasia at 856-779-3223, email@example.com, or @PhilAnastasia on Twitter. Read his blog, "Jersey Side Sports," at www.philly.com/jerseysidesports