LeGarrette Blount ran for 166 yards and four touchdowns to spark the Patriots over the Indianapolis Colts 43-22 while Marshawn Lynch ran for 148 yards and two touchdowns as Seattle beat the New Orleans Saints 23-15.
New England next face the winner of Sunday's Denver-San Diego game in the American Conference final on January 19 for a Super Bowl berth.
The other spot in the February 2 NFL championship spectacular in suburban New York will be decided in the National Conference final the same day between Seattle and the winner of Sunday's San Francisco-Carolina matchup.
New England's defence intercepted four Andrew Luck passes while the offence grinded out 234 rushing yards.
"We've done a really great job of that," said Patriots quarterback Tom Brady.
"We have been a great running team. It has really been a strength.
"If we don't make mistakes we're tough to beat."
Brady completed 13-of-25 passes for 198 yards in his record-stretching 18th career playoff victory while coach Bill Belichick's 19th playoff win put him one shy of former Dallas coach Tom Landry's all-time record.
Luck was 20-of-41 for 331 yards but could not rally the Colts once New England jumped ahead early.
"We couldn't get over the hump and we paid for it," Luck said.
The Seahawks denied a late comeback bid by Saints quarterback Drew Brees and moved within one victory of their first trip to the Super Bowl since losing to Pittsburgh in 2006.
"It's only a first step but it's a hell of a first step," said Seahawks coach Pete Carroll.
Seattle, who went 13-3 this season to earn a playoff home-field edge and a first-round bye, will host the winner of Sunday's Carolina-San Francisco game in the National Conference final.
"We haven't done anything yet," Seattle quarterback Russell Wilson said.
Blount scored on three 2-yard runs in the first half, sparking New England to a 21-7 lead, and had a 73-yard touchdown run in the fourth quarter.
Luck threw touchdown passes of 35 and 38 yards to LaVon Brazill.
The Colts used two Adam Vinatieri field goals and a safety to pull within 21-15 but New England's Stevan Ridley had touchdown runs of 1 and 3 yards in the second half and Indianapolis could not close the gap.
At Seattle, the Seahawks took support from their notoriously loud fans and improved to 16-1 at home in the past two seasons.
Lynch, who carried the ball 28 times, scored on a 15-yard run in the second quarter and Steven Hauschka kicked field goals of 26, 38 and 49 yards to give the Seahawks a 16-0 halftime lead.
That was the same score entering the fourth quarter, the Seahawks' NFL-best scoring defence leaving the Saints without a point through three quarters for the first time since 2002.
The NFL announced on Saturday that the Red Hot Chili Peppers will join Bruno Mars as halftime entertainment at the Super Bowl.