SEATTLE - Aleighsa Welch, Tiffany Mitchell and South Carolina made certain there was no second-round upset this time for the Gamecocks.

Welch had 21 points and 11 rebounds, Mitchell 20 points and 10 boards, and top-seeded South Carolina used its size and athleticism to overwhelm No. 9 seed Oregon State, 78-69, on Tuesday night in the NCAA tournament.

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.

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.

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.

Oregon State's rally came far too late. Oregon State was 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.

South Carolina slogged through its first-round victory over Cal State Northridge, letting the No. 16 seed hang around for longer than anyone anticipated.

There was no sluggishness from the Gamecocks on Tuesday. They imposed their advantages from the start.

While not the tallest player on the court, Welch dominated the interior in the first half with 12 points and five of her seven rebounds coming on the offensive end. South Carolina then started the second half with a pair of deep 3s from Mitchell and Asia Dozier and a nine-point halftime lead was pushed to 15. Oregon State made one run, getting within 10 on Hanson's basket with 10:25 left. After a South Carolina turnover, Wiese was unable to hit an open 3 and Mitchell answered with her second 3 of the half to push the lead back to 13.

The Gamecocks wanted to limit Wiese's impact offensively and were 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.

By midway through the second half, Wiese seemed resigned to letting her teammates take shots until the closing minutes when the Beavers tried to rally.