Not many public courts near 8th and Market.
Nearest might be Turk and Buchanan, where you might get shot at or harassed by our brothers. Or at least get your car checked out. That playground is fine for rec softball, where 40 guys show up, but for two guys in tennis, maybe not the smartest move.
Private clubs at 5th & Brannan, foot of Market, and Bush & Steiner, but they won't let you in without a member partner.
GoldenGatePark is your call.