By Mary Milliken and Steve Keating

RIO DE JANEIRO (Reuters) - The United States beat France 86-67 on Thursday to secure their ninth trip to the women's Olympic basketball final, where they will meet medal game debutantes Spain.

In pursuit of a sixth consecutive gold medal, the world number one United States encountered more resistance from feisty fourth-ranked France than they have from other opponents here. They also suffered with the absence of team anchor Sue Bird, benched by a sprained knee.

The Americans' 19-point margin of victory was far below their average of 41.7 in their first six games in Rio, and at halftime they were ahead by a mere four points.

Bird, a veteran guard and captain, helped the team recover for the second half with a calm talk in the locker room.

"You forget how much Sue does for this team," said Diana Taurasi, the top game scorer with 18 points.

"We have so many scorers and she finds a way to get everyone in their spots, to get everyone touches and obviously she sacrifices for that."

The 35-year-old Bird said there is no guarantee she will be able to return for the final Saturday.

"She obviously wants to play, I want her to play. We are a much better team when she is on the floor," said coach Geno Auriemma.

In the other semi-final, Spain eased past Serbia 68-54, and, while delighted to be assured of their first women's Olympic basketball medal, they were sanguine about their chances to defeat what they consider a "dream team".

"It is impossible to beat them. It will be very, very complicated," Spain forward Laura Nicholls told reporters.

"But this is a final you have to see, you have to play and enjoy, enjoy, enjoy. I'm sorry, but you have to be realistic."

The U.S. team extended their Olympic winning streak to 48-0 since 1992.

'DIFFERENT GAME SATURDAY'

Spain, who had edged Serbia 65-59 in their opening group game, dominated from the start, storming to 20-9 lead in the opening quarter and never trailed the rest of the way.

Alba Torrens and Astou Ndour led Spain with 14 points apiece while Nicholls chipped in with 13.

"I cannot explain how we won with such a big margin after suffering so much in quarter-finals," said Spain's Laura Quevedo. "This is history for us. We had never made it past the quarter-finals and now we are in the final."

The American women said they are taking Spain seriously even though they defeated them by a resounding 103-63 in their second game here.

"We've got to be serious and really get this because it is going to be a different game this Saturday," said forward Angel McCoughtry.

The heavily favored American men will also take on Spain in a hot-ticket semi-final on Friday between the two top teams in Rio.

France, which lost the gold to the United States in London 2012 86-50, will play for the bronze against Serbia.

"We'll go for it. We have revenge to take against Serbia who beat us last year at the European championship final, so we really want to go get this medal," said captain and center Isabelle Yacoubou.

(Reporting by Mary Milliken and Steve Keating; Editing by Frank Pingue and Andrew Hay)