Sharapova beat Victoria Azarenka 6-4, 6-2 in the second semifinal, a day after Azarenka had made sure of finishing the year as the top-ranked player.
Sharapova will finish No. 2 even if she beats Williams in Sunday's final.
Williams was never challenged by the fourth-ranked Radwanska apart from dropping serve in the fourth game of the first set.
Radwanska spent more than eight hours on court in her previous three matches, while Williams required less than five to win her three round-robin matches. The difference showed, although Williams did not think that was the case at the start of the match.
"I definitely had an idea that she was tired," Williams said.
To reach the semifinals, Radwanska beat Sara Errani in a match that took 3 hours, 29 minutes -- the longest best-of-three-sets match in WTA Championships history.
"I told her (after the match) it was awesome that she played so well and played through another match after playing a good eight hours," Williams said.
Sharapova, who rose to No. 1 briefly after winning the French Open, clinched the match against Azarenka when she held to lead 5-1 in the second set after prevailing in a 15-minute game that went to nine deuces, the last one on a double-fault by Sharapova.
Azarenka, of Belarus, ended her best year on a losing note but she became the first woman to reach the $7-million mark in prize money for a year.
Sharapova is seeking her second title in the WTA Championships after her first in 2004. The Russian had lost her last two matches against Azarenka, in the semifinals of the U.S. Open and the final at Beijing. Azarenka leads the series 7-5.
Williams has dominated the second half of the year, winning Wimbledon, an Olympic gold medal in singles, and the U.S. Open. She has won 47 of her past 49 matches and will finish the year ranked No. 3.
The American is seeking her third title in the year-end tournament that brings together the top eight players in the world. She won her debut in 2001 and again in 2009. She has six victories this year and is the only player to win titles on all three surfaces: clay, grass and hard.
Williams took a six-week break after winning the U.S. Open in September but has looked sharper with every match. She now holds a 4-0 record against Radwanska.
Williams is 8-0 in 2012 against the other semifinalists at the tournament.
The American has won 18 of her last 19 meetings against opponents ranked in the top four, including 12 straight.