Two-time Olympic champions May-Treanor and Walsh saw off a fierce Chinese challenge in their semi-final, bringing closer the prospect of a third gold.
The only team of either gender to defend an Olympic beach volleyball title since the sport's Games debut in Atlanta in 1996, May-Treanor and Walsh won gold in Athens in 2004 and in Beijing four years later.
"We have a goal that we really want to accomplish and we're planning on accomplishing it," said Walsh, 33, who has had two babies since she her last Olympic final.
Ross and Kessy scored the biggest shock in the event by beating favourites Brazil's Juliana Felisberta and Larissa Franca.
"To be in the position we're in now is a dream coming true," said Ross.
The number two US team have spent most of their careers in the shadow of May-Treanor and Walsh. Yet Ross and Kessy were world champions in 2009, when May-Treanor and Walsh were not playing.
On Tuesday, they beat the Brazilians by two sets to one on a score of 15-21 21-19 15-12.
It was the first major upset of the women's event and a crushing outcome for Juliana and Larissa, who have won every trophy available except for an Olympic medal.
Juliana missed out on the Beijing Games in 2008 with a knee injury. She said before the London Games that her dream was to face May-Treanor and Walsh in the final. Instead she and Larissa will play Zhang Xi and Xue Chen in the bronze medal match.
The Chinese pair, bronze medallists in Beijing, have won a string of trophies since.
Against May-Treanor and Walsh they raced to a 13-7 lead in the first set with a barrage of spikes. Briefly, it looked as though the defending champions had finally met their match.
"They do that, they jump on teams. I was hoping that we would be the ones jumping on them but we got knocked back," said Walsh.
The Californian pair kept cool under pressure and raised their game, shocking the Chinese with a string of blistering attacks of their own to draw level at 13-13.
From then on the match was too close to call, and with a two-point advantage required to win both sets went past the usual 21-point scoreline with the Americans winning 22-20 22-20.
"They started firing back with more artillery. We put a stop to that and got them at the end, thank goodness," said Walsh, putting it down partly to experience.
"We've had 11 amazing years together. We've been in every single situation. That was not a fun situation to be in but we have survived it before and we did it again tonight."
In the other semi-final, played under floodlights in the pouring rain, Larissa and Juliana made a strong start and did not look in danger in the first set.
They took an early lead in the second set, but after a time-out in the middle of that set Ross and Kessy found new inspiration and the match turned.
"I went and got gum in the time-out because I forgot gum, and I always play with gum, and then we started to win. It felt like something was off before I put the gum in," said Kessy with a laugh.
The final is scheduled for Wednesday. Britain's Prince Harry, second son of heir-to-the-throne Prince Charles, is expected to attend, which will please Ross and Kessy.
Harry played a well-publicised game of beach volleyball during a recent trip to Brazil and Ross and Kessy commented during a pre-Games press conference that they were keen to see him in the stands.
The success of the American women contrasts their male counterparts as US defending champions Todd Rogers and Phil Dalhausser were knocked out by an Italian pair in the round-of-16, while in-form compatriots Sean Rosenthal and Jake Gibb lost to a Latvian team in the quarter-finals.