In 1991, for the fourth time, Metallica played as part of the Monsters of Rock festival tour. The last concert of the tour was held on September 28, 1991, at Tushino Airfield in Moscow; it was described as "the first free outdoor Western rock concert in Soviet history" and was attended by an estimated 150,000 to 500,000 people. Some unofficial estimates put the attendance as high as 1,600,000. The first tour directly intended to support the album, the Wherever We May Roam Tour, included a performance at the Freddie Mercury Tribute Concert, at which Metallica performed a short set list, consisting of "Enter Sandman", "Sad but True" and "Nothing Else Matters", and Hetfield performed the Queen song "Stone Cold Crazy" with John Deacon, Brian May and Roger Taylor of Queen and Tony Iommi of Black Sabbath. At one of the tour's first gigs the floor of the stage collapsed. The January 13 and 14, 1992, shows in San Diego were later released in the box set "", The show was cut short shortly after this accident, so that Guns N' Roses began their concert to malicious reactions from fans. Newsted said Hetfield's skin was "bubbling like on "The Toxic Avenger"". The tour recommenced on August 25 in Phoenix, and although Hetfield could sing, he could not play guitar for the remainder of the tour. Guitar technician John Marshall, who had previously filled in on rhythm guitar and was then playing in Metal Church, played guitar for the recovering Hetfield. Brazilian musician, Andreas Kisser from Sepultura was initially considered for play the tour, but Marshall finally was chosen.