SINGAPORE—Just last night, Serena Williams said that if she won three games against Simona Halep in today’s final, she would be very pleased. Sounds strange, but remember, Halep crushed Williams in their round-robin meeting on Wednesday, giving up just two games.

Today, she won only three games—Halep, that is. Williams reversed course in the rematch, crushing Halep, 6-3, 6-0, to win the WTA Finals.

Once again, Williams stepped up to the plate in a very significant contest and rose to the challenge, overwhelming her young and talented opponent. Halep looked impressive all week, but had nowhere to go today, as the 33-year-old world No. 1 started quickly.

Fired up, Williams held in the first game, pumping up her feet, dancing around, and screaming after her winners. Halep, quick and adept from inside the baseline, was surprised that Williams refused to engage in long rallies and instead chose attack immediately. Williams rushed the net off deep groundstrokes and polished off forehands and backhands to the corners, of all which Halep couldn't handle.

Still, Halep hung in, only down 4-3, but she was rushing and was badly missed her backhands wide. At 5-3, Williams took her gloves off. She hit two gigantic serves that Halep couldn't touch en route to taking the set. It was surely satisfying for Serena, who went 11 of 14 on net points—a high conversion percentage for her on slow courts.

In the second set, Serena was locked in while Halep fell apart. Williams stepped in and hit as hard she could on second serves. Halep had no solution to Serena’s rather simple formula: Hit winner after winner. On her second match point, Williams was ready: She hit a reasonable serve and moved forward for Halep’s high return. Williams backed up a little bit, jumped up in the air, and slapped a crisp overhead for the final winner.

The Romanian smiled very broadly and shook her head as if saying, ‘How did I actually win this week?’ As Halep said later, she couldn't manage the ball, she was tired, and her arm was sore.

“I had to play more Serena-style tennis and just do what I do best—enforce myself,'' Williams said. “That's what I was trying to do.”

The result means Williams, who nearly pulled out of this tournament with leg trouble, has won the last three WTA Finals, a testament to her stamina at the end of nearly year-long seasons. Seven other players came to Singapore and hoped they could win the trophy, a mix of veterans (Maria Sharapova, Ana Ivanovic), experience (Caroline Wozniacki, Petra Kvitova, Agnieszka Radwanska), and kids (Halep, Eugenie Bouchard). But the oldest—and most decorated—of them all was triumphant again.

Currently holding 18 Grand Slam titles, Williams could pass Chris Evert and Martina Navratilova next year, and who knows where she can go from there? What do know, and what she showed in Singapore, is that she is ageless.

“I do have goals, but I always keep them to myself,” Williams said. “No disappointments.”