[C. W. Fields]

Stosur it is. SS leads H2H 3-2 and 2-0 on HC. Their last match was YEC 2010. The decisive factor in that match was BP conversion. As I recall, SS was 2/3 while Caro was 0/6. Neither of them have had a stellar season: Caro sliding from #1 to #11, SS from #6 to #9. Their W/L is almost similar: Caro 45-20, SS 44-21. Caro has won 1 title this seaso, SS zero. But SS is 10-4 in GSs while Caro is only 6-4.

An important factor is how well Caro can handle Stosur's kick serve. It didn't look as lethal today as on other occasions (no aces). Another factor is Stosur's net play. The surface is so slow I don't think she can hit through Caro consistently, but I can well imagine her hitting good approach shots which Caro can only just get to and then move to the net. Caro's passing shot must be spot on, her serve must be better than today and she must be ice cold on BPs if she wants to win this match. Caro has been on court nearly 5 hours the last two days so Stosur is probably better rested.
Stosur is 55/45 favorite in my eyes.

Edit: If Caro wins tomorrow and manages 4 wins in Sofia, it'll be the 5th season in a row she has won 50 matches or more.