Thanks Hawkeyes for the response, I've decided on this:
3* Serena Williams -225 W +3
This is a wager for 6.75 units to win 3 units. Already having a 2* future bet on Sharapova at +465 gives me some cushion for this match tonight. If Williams wins the match, it covers the 2* future plus profit 1 unit. If Sharapova wins the match, I'm out 6.75 units but cover it with the 9.3 to profit 2.55 units.
2* Maria Sharapova +465 (bookmaker) L -2