Last-place Boston took the game to the Portland Thorns on Sunday.
But the Breakers don't have Christine Sinclair.
Sinclair scored her eighth goal of the season in the 72nd minute as the Thorns clinched a home playoff game with a 1-0 win at Harvard University's Jordan Field.
The win means the Thorns can finish no worse than second in the final standings.
Portland (13-5-4, 43 points) are seven points clear of third-place teams Chicago and Orlando. The Thorns, trail first-place North Carolina by two points. North Carolina has three games left, Portland has two.
Sinclair put away a chance created when Hayley Raso dribbled to the top of the 18-yard box. Three Boston defenders reacted to Raso's run and she pushed the ball to her right into space where Sinclair ran onto the ball for the finish.
Sinclair now is the Thorns all-time leader in goals with 31 over the club's five seasons. She moved ahead of Allie Long with Sunday's goal.
Boston outshot Portland 17-3 including 5-2 on goal.
Among the better chances for the Breakers was a free header for Julie King in the 58th minute that missed high.
Thorns goalkeeper Adrianna Franch stopped four shots to earn her 10th shutout of the season, which ties her for the NWSL single-season record. Nicole Barnhart had 10 shutouts for FC Kansas City in 2013.
The NWSL is idle next weekend for an international break. The Thorns will visit Orlando on Sept. 23 then play host to Chicago on Sept 30.
The semifinal playoff match will be Oct. 7 at Providence Park. The league championship game is scheduled for Oct. 14 at Orlando.
The Thorns were without central defender Emily Menges, who was with her family following the death of her brother. Celeste Boureille stepped into the back four for Portland.