Serena Williams demolished Poland’s Agnieszka Radwanska on Saturday to reach her fifth WTA Championships final.

Williams improved her head-to-head score against Radwanska to 4-0 after completing an easy 6-2, 6-1 win in which she landed 39 winners.

In the final of the $4.9 million season-ending event, Williams will face either world No. 1 Victoria Azarenka, or Russia's Maria Sharapova.

Radwanska failed to offer much resistance to world No. 3 Williams after coming through an epic match Friday against Sara Errani in the longest best-of-three match in WTA Championships history.

"Those last two matches really killed me," Radwanska said. "I really tried today and I really wanted to run, but my legs just didn't. This is the kind of tournament where you have to be fit every match to win.”

The Polish 23-year-old kept up with Williams before the American embarked on an eight-game winning streak at 2-2 in the first set.

Radwanska's defeat means that she finishes the season as world No. 4 after winning three titles in 2012.

"To be honest it was a really great year, especially that I was really close to No. 1,” said Radwanska, who reached the Wimbledon final this year.

“I was No.2 for a couple of weeks after my first Grand Slam final."

Williams has won the tournament twice, beating Lindsey Davenport in 2001 final and her sister Venus in 2009.