Simona Halep ended Serena Williams's 16-match unbeaten streak at the WTA Finals in emphatic fashion by running out 6-0 6-2 victor in Singapore.

The American's winning run had seen her triumph at the season-ending event over the last two years, but she was outgunned from game one by the fourth seed.

Halep needed only one opportunity to break the world number one in consecutive service games from the beginning of the encounter, and surrendered just one point while holding comfortably to storm into a 5-0 lead.

The Romanian completed a shock first-set bagel in just 21 minutes and stayed on the front foot with a break in the first game of the second, before Williams finally got on the board in the match's ninth game.

Three of the next five games went to deuce, but the 18-time Grand Slam champion simply could not live with Halep, who broke once more on the way to securing an impressive straight-sets triumph.

The result - Williams's worst Tour Finals performance since 1998 - means that Halep has won both of her Red Group matches so far and will qualify for the last four if Ana Ivanovic beats Eugenie Bouchard later today.