His career began to rapidly climb from this point. In the later half of 2003 Tanahashi captured the IWGP U-30 Openweight Championship and the IWGP Tag Team Championship, the latter on two separate occasions. In August 2004 he made it to the finals of the G1 Climax and on January 4, 2005 he headlined the show in Tokyo Dome in a match, where he dropped the U-30 Openweight Championship to Shinsuke Nakamura. In 2005, Tanahashi also made an appearance in the 15th Sasuke competition on July 20. He was eliminated in the first round. On June 18, Tanahashi defeated Toru Yano in a tournament final to regain the U-30 Openweight Championship. He would go on to defeat Masahiro Chono by Sling Blade in the first round of the New Japan Cup (determining a No. 1 contender against IWGP Heavyweight Champion Brock Lesnar) on April 16, 2006, in Korakuen Hall and Hiroyoshi Tenzan by Dragon suplex hold in the second round on April 25 in Omuta Citizen Gymnasium before falling to Yuji Nagata's wrist-clutch Exploder in the semifinal on April 29 in the Tottori Industrial Gymnasium. Tanahashi also defeated Tiger Mask on May 13 in the first ever main event of the new Wrestle Land brand (a promotional experiment by New Japan with "sports entertainment"), debuting a modified side buster finish. On June 7, Tanahashi vacated the U-30 Openweight Championship in order to fully focus on his upcoming match against Brock Lesnar for the IWGP Heavyweight Championship.