SEATTLE (AP) -- Gone were the jitters and apprehension from two days earlier when it was fair to question if South Carolina deserved a No. 1 seed. The Gamecocks left no doubt Tuesday night leaving Oregon State bruised and erasing all those memories of being upset in the second round a year ago. ''We knew from the get go we wanted to be aggressive and not back down to their size,'' South Carolina's Aleighsa Welch said. ''What we wanted to emphasize was to go right through them. Not try and go around them or try to go over the top but go right through them, through their chest. I think it paid off.'' Welch had 21 points and 11 rebounds, Tiffany Mitchell scored all 20 of her points in the second half and grabbed 10 rebounds, and top-seeded South Carolina used its size and athleticism to overwhelm No. 9 seed Oregon State 78-69. A year after getting upset by a 12 seed in the second-round of the tournament, the Gamecocks are headed to the Sweet 16 for the second time in three years under coach Dawn Staley. South Carolina (29-4) will face No. 4 seed North Carolina in the Stanford Region semifinals on Sunday. The Gamecocks lost to the Tar Heels 74-66 in December. ''This is a matchup we wanted to see later on in the season,'' Welch said. But that meeting against the Tar Heels will come on Stanford's home court with a potential meeting coming against the Cardinal in the regional final. Staley didn't hide her displeasure with possibly having to play Stanford on its home floor. ''I think it (stinks). I do,'' Staley said. ''The people out at Stanford appreciate good basketball. I know that because we have played out there on a number of occasions but when it's the Final Four at stake I think it should be at neutral sites.'' South Carolina slogged through its first-round victory over Cal State Northridge, letting the No. 16 seed hang around for longer than anyone anticipated. That sluggishness was replaced with an aggressiveness that Oregon State couldn't match. Welch's quickness around the rim had the Beavers flustered. Whether she was following up misses or getting open for her own shot, Oregon State struggled to slow the junior forward. Welch made 8 of 11 shots and had six offensive rebounds. South Carolina also outscored Oregon State 34-9 at the free-throw line, many of those coming in the final two minutes. Mitchell was 10 of 11 at the line. ''We're not satisfied with making it to the Sweet 16,'' Welch said. ''We want to make a deeper run in this tournament. We can't get caught up in the fact we're here. This isn't where we want our season to end.'' Jamie Weisner led Oregon State (24-11) with 21 points, including five 3-pointers, before fouling out. Gabriella Hanson added 16 points. Leading scorer Sydney Wiese, coming off a career-high 26 in the first round win over Middle Tennessee, suffered through one of the worst nights in her freshman season. She missed her first 12 shots before hitting a 3-pointer with 1:30 left that cut the South Carolina lead to 66-57. Wiese finished with eight points. The Gamecocks ability to limit Wiese's impact offensively was probably more successful than anticipated. Wiese was 0 of 7 shooting in the first half and rarely could get Oregon State's offense started without harassment from Khadijah Sessions, Olivia Gaines or Mitchell. ''That's a hard feet. Sydney Wiese is a really good player. She really is,'' Staley said. ''She can stroke it, find other teammates, controls the tempo I thought we did a terrific job of controlling all those aspects of her game. The only reason why we win this game is we don't allow her to get her average and allow her to play comfortably in the flow of their offense.'' Oregon State's rally came far too late. The Beavers were unable to join a select list of No. 9 seeds to reach the round of 16. Notre Dame and Arkansas in 1998 and Michigan State in 2009 remain the only No. 9 seeds to reach the Sweet 16. ''The interesting thing about this team in just watching, there's times when we think, 'Oh, we can play with that team,' and then as soon as you get close, they hit a different gear,'' Oregon State coach Scott Rueck said of the Gamecocks. ''I think that's what's special about this group.''