Oneonta (8-2-3, 4-0-1), which extended its unbeaten streak to 10 games, faced a 1-0 deficit before tying it in the 78th minute. Williams scored the equalizer, heading a cross from Jake Sutherland into the lower-left corner.
Plattsburgh (8-3-1, 2-2) took a 1-0 lead in the 11th minute, when Tom Greene converted a pass from Fabian Chaparro.
Vincent Pellegrino made seven saves for the Red Dragons. Counterpart Mitchell Jordon stopped nine shots.
Oneonta hosts Potsdam at 1 p.m. Saturday.
Oneonta State advanced to the SUNYAC Tournament semifinals for the third straight season with a 5-0 sweep of Fredonia in the quarterfinals at the Binghamton Tennis Center.
The Red Dragons will face Geneseo at 11 a.m. Saturday in a semifinal. The winner will play for the conference’s automatic berth to the NCAA Division III Tournament on Sunday morning.
Friday’s quarterfinal ended before four singles matches were completed as Oneonta (8-3) earned its fifth point during singles play.
Lauren Skolnick and Brynn Sussman won at first doubles, 8-0; Emily Maggiore and Colette Lankowicz contributed a 9-7 win at second doubles; and Olivia Stauffer joined Abigail Smith for an 8-4 win at third doubles.
Sussman and Lankowicz won in straight sets at first and fourth singles, respectively, to end the match.