The BNP Paribas WTA Finals Singapore presented by SC Global witnessed a major upset on Wednesday, as world No. 1 Serena Williams suffered a rare thrashing, falling 0-6, 2-6 to a scintillating Simona Halep at the Singapore Indoor Stadium.

While there were concerns over Williams' health and knee issues before she came to this prestigious season-ending event, no one could have anticipated how badly the American fared against her Romanian opponent, who had not beaten her in three past encounters.

However, Halep took control from the get go, breaking Williams twice for an early 3-0 lead and, before Williams had time to assess the situation, the world No. 4 kept up her counter-punching pressure to break the American for a third time. Halep then served out the first set, as the packed crowd at the Indoor Stadium witnessed a rare "bagel" for Williams.

When Halep broke Williams again to begin the second set, frustration began to show for the 33-year-old American. While the 18-time Grand Slam winner finally won a game, she had no answer for the incessant returning prowess of her 24-year-old rival, and meekly surrendered the second set 2-6.

With the win, Halep leads the Red round-robin group with two wins, while Williams has a win and a loss.