(CNN) -- Four days after suffering a humbling defeat to Simona Halep, Serena Williams turned the tables on the Romanian to win a fifth WTA Finals title with a crushing straight sets 6-3 6-0 victory in Singapore Sunday.

Williams had called her 6-2 6-0 reverse in round-robin group play Wednesday "embarrassing" and she set about putting the record straight in the title match.

Fourth-ranked Halep began where she left off to take an early 2-1 lead with a break of service, but this was a very different Williams on the other side of the net.

The world number one immediately broke back and a further break to lead 5-3 was the start of eight games in a row without reply.

A total of 26 winners flowed from the racket of the 33-year-old American veteran -- who was winning the end of season crown for the third straight year -- the first player since Monica Seles in 1992 to achieve the feat.

"She was playing so well at the beginning and I told myself to just relax and once I did that I started playing better and making my shots," Williams told the official WTA website.

"I lost to her a couple of days ago so I knew she was capable of playing really well, but I knew I had to play better if I wanted to win.

Williams' participation in the WTA Finals had been in doubt after she pulled out of a warmup tournament in China with a knee injury.

1. Who won her fifth WTA title?
2. What was the score?
3. Who did she play?
4. Why did she not play in China?
1. Serena Williams
2. 6-2 6-0
3. Halep
4. knee injury