Summerfest rounds out its Marcus Amphitheater with two more big announcements.

Alicia Keys headlines the Marcus Amphitheater at 7:30 p.m. Tuesday, July 1. An opening act will be announced soon. Her 2007 release "As I Am" features the single, "No One," which was one of the fastest moving and highest charting track on Billboard's Hot R&B / Hip-Hop Songs chart, and RCA's first-ever gold-selling ringback tone, with 500,000 downloads. Tickets go on sale at noon April 12 and will be available for $80, $70, $55 and $35.

John Mayer headlines the Amphitheater at 7:30 p.m. July 2 with opening acts Colbie Caillat and Brett Dennen. The five-time Grammy winner's most recent single "Say," was featured in the film "The Bucket List," and was packaged as part of the deluxe re-release of his "Continuum" in November 2007. Tickets go on sale at 10 a.m. April 19 at and cost $66, $56, $43.50 and $31.

Summerfest 2008 takes place June 26-July 6, from noon until midnight daily.