Serena Williams all but booked her place in the semi-finals of the WTA Championships courtesy of a two-set victory over Li Na in Istanbul this evening.

There was very little to separate the two player during the opening set and as a result a tie-breaker was required to determine a winner.

Wimbledon champion Williams took control of the break early on with a string of forehand winners. She eventually ran out a 7-2 victor against her Chinese opponent.

The second set proved to more routine for Williams, who broke Li on two occasions.

Despite being broken back to reduce the arrears to 5-2, Williams served for the match to win through 6-3.