Serena Williams suffered her heaviest defeat since she was 16 years old. Serena was routed 6-0, 6-2 by Simona Halep at the WTA Finals in Singapore yesterday. It was arguably the worst performance of the18-time Grand Slam champion's career. The last time Serena managed to win just two games in a WTA Tour or Grand Slam match was in 1998 when she was a newcomer to the tour, known best as Venus' little sister. The loss in the round-robin match provided a shuddering halt to Serena's 16-match winning streak at the year-end championships, where she is the two-time defending champion. The fourth-ranked Halep had never before beaten a player ranked in the top 3, and in her two matches against Serena last year, she only won a total of seven games. For a player, who was ranked No. 47 two years ago, yesterday's win over No. 1 Serena represented a new career high.Halep, runner-up at this year's French Open, beat Eugenie Bouchard in her opening match and moves to 2-0 in the Red Group. Serena slips to 1-1 and will face Bouchard next. AnaIvanovic beat Bouchard in yesterday's other game, winning 6-1, 6-3. Ivanovic is 1-1 and Bouchard fell to 0-2. Going into the last round-robin matches in the Red Group, Halep is all but certain of a semi-final berth, needing to win just one game against Ivanovic, while Bouchard has only a very slim mathematical chance of progressing.Serena's defeat jeopardized her chances of finishing the year as the top-ranked player. Had she won yesterday, the only way she could have lost the No. 1 spot is if Maria Sharapova won the title. Now, just reaching the final may be enough for the Russian, depending on the result of Serena's match against Bouchard today. In today's matches in the White Group, Carolina Wozniacki meets Agnieszka Radwanska in a clash of two players who won their opening matches, while Sharapova faces Petra Kvitova.