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 in Rio.

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.

Diana Taurasi powered the Americans with a game-high 18 points.

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."

American captain Sue Bird did not suit up after spraining her right knee on Tuesday against Japan. It is not clear if the veteran guard will be ready to for Saturday's final.

The American women extended their Olympic winning streak to 48-0 since 1992.

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 believe we are in the final," said Spain's Laura Quevedo. "I cannot explain how we won with such a big margin after suffering so much in quarter-finals.

"This is history for us. We had never made it past the quarter-finals and now we are in the final."

(Editing by Frank Pingue)