I have had the opportunity to get out to River Park tennis courts twice already this season. If you are looking for availibility of courts within realativly close proximity to downtown...this may be your place.
River Park is found close to the junctions of W Foster and N Francisco Avenues, just to the west of the North Branch of the Chicago River. The park can be a bit tough to reach in traffic since it boarders the river and thus, there are not many ways to cut across the river and avoid Kedzie traffic.
The courts themselves are in really rough condition, about a 3 out of 10. There are substantial cracks in all courts. And really all but one of the five courts will cause "clay like" strange bounces due to the extensive and large cracks. We normally just make the akward bounces part of our game...everyone is playing on the same courts. The nets are about the only thing that is in worse shape than the courts. It appears as of Spring 2013 the Chicago Parks Department did replace one of the nets, but they neglected to put a center strap in any of the five nets (photo is from a few years ago) So you are going to be playing on nets that average 3 feet 6 inches in the center...tough play if you hit flat. Of the courts without new nets, one net is OK...the others are comically horrible. Three of the nets are compleatly distroyed with holes so big you won't be sure if a serve was clean, or it just went straight through without hitting any net.
The lights are pretty good at River Park in the three center courts. The CPD appears to have replaced some light bulbs over the winter as they no longer have that creepy yellow look they had last year. But there are still several lights that are not working or burnt out. So if you are going to play in the evening, try to grab one of the three courts in the center. The two courts on the edges are very dark. If you play early or late in the season it is important to note that the lights do not come on until well after dusk. As of May 13th there was about a 30 min period where it was too dark to continue with a competitive match before the lights turn on.
Map to River Park: https://maps.google.com/maps?hl=en&safe=off&ie=UTF-8&q=river+park+chicago&fb=1&gl=us&hq=river+park&hnear=0x880e2c3cd0f4cbed:0xafe0a6ad09c0c000,Chicago,+IL&cid=0,0,3519716325774025554&ei=4fSTUdebN8blyQG934HQDA&sqi=2&ved=0CLMBEPwSMAA