Agnieszka Radwanska into WTA Championships semi-finals

Agnieszka Radwanska has made it through to the WTA Championships semi-finals in Istanbul after coming from a set down to defeat Sara Errani.

The match was the longest in the tournament's history as the two players battled it out on court for three hours and 31 minutes.

The Italian won an error-strewn first set 8-6 in the tie-break before Radwanska levelled the contest with a 7-5 triumph in the second.

Errani tired in the third and it was the Polish world number four who sealed the win 6-4.

Radwanska will now meet Serena Williams for a place in Sunday's final.