Serena enters final
Tennis Foreign round-up:American to face Sharapova for title
Williams roared through her round robin group, winning all three matches, and proved unstoppable again in a 6-2, 6-1 victory that set up a final against Maria Sharapova.
Sharapova beat Victoria Azarenka, who will finish the year as No.1 in the rankings, with similar ease 6-4, 6-2.
Both are yet to drop a set in the showpiece tournament.
World number three Williams rattled off 39 winners against the leg-weary Radwanska.
Radwanska, who took three hours and 29 minutes to beat Sara Errani on Friday, just about stayed with Williams until 2-2 in the first set but her resistance quickly crumbled against the 15-time Grand Slam champion.
Azarenka, who had sealed the year-end number one ranking by advancing to the semifinals, lacked her normal power and foot speed and was no match for Sharapova who broke the Belarussian's serve twice in the first set with some vicious returns.
The four-time Grand Slam champion raced out to 4-0 lead in the second set, fought tooth and nail to win a 24-point game to go ahead 5-1, and then easily held to win the match.
Federer, Del Potro in final
Roger Federer and Juan Martin del Potro will meet on Sunday for the first time since their epic Olympic Games clash after both beat French opponents to reach the final of the Basel Open.
Argentine Del Potro overpowered Richard Gasquet 6-2, 6-2 while world number one Federer met more resistance before beating Paul-Henri Mathieu 7-5, 6-4.
Nicolas Almagro's chances of making next month's World Tour Finals in London were dealt a blow when he was beaten 6-3, 7-5 by top seed and Spanish compatriot David Ferrer in the quarterfinals of the Valencia Open on Friday.
Ferrer nets faces Ivan Dodig after the Croatian qualifier upset compatriot and seventh seed Marin Cilic 6-2, 7-6.
Alexandr Dolgopolov of Ukraine beat Marcel Granollers 6-0, 6-2 to set up a semifinal against Juergen Melzer.
Austrian Melzer saw off promising Belgian David Goffin, ten years his junior, 7-6, 6-4.