American Williams was made to wait before speeding towards a first Olympic women's singles gold medal with a 6-1 6-2 thrashing of Victoria Azarenka after Sharapova won an all-Russian clash against Maria Kirilenko.
Williams, trying to complete her full set of major honours, needed just 63 minutes to destroy the Belarussian world No.1 - making up for a long wait to get on Centre Court after Roger Federer's epic semi-final win against Juan Martin del Potro.
"I warmed up probably five or six times. But I can wait with the best of them," Williams, whose later women's doubles semi-final with sister Venus was postponed, told reporters.
"I have been waiting my whole life.
"I have a lot of patience - that's the only thing I have patience with, which is kind of ironic."
Russia's opening ceremony flag bearer Sharapova won 6-2 6-3 to reach the Olympic final on her first Games appearance.
Azarenka and Kirilenko will now play off for a bronze medal.
Williams has been unstoppable on grass this summer.
First she claimed a fifth Wimbledon title to secure a 14th grand slam title. Now she is on the verge of an Olympics singles gold to go with the two she won in doubles with Venus.
The 30-year-old Williams, seeded four, has conceded only 16 games in five matches to reach the final.
Standing in her way in Saturday's final will be Sharapova, who is hoping to emulate fellow Russian Elena Dementieva's feat in Beijing four years ago.
"I really look forward to playing Maria," Williams, who lost the 2004 Wimbledon final to Sharapova, said.
"I haven't played her in a while. We always have good matches. It will be interesting. It will be a good battle."
Sharapova, wearing a Russian red shirt, was not quite as dominant against Kirilenko but still moved into the final without too many alarms.
"Well it's incredible, not only to be part of this event and be an Olympian, but to put yourself in with an opportunity to go for gold. It's a really nice feeling," Sharapova told reporters.
"I've been really fortunate to win all the grand slams and obviously this is my first Olympics so I'm just thrilled to be in the finals."
Sharapova was made to fight by Kirilenko in the second set but claimed victory with a volleyed winner on her second match point before blowing kisses to the fans watching on Court One.
- Maria Kirilenko