However, a succession of half-chances for Real helped them gain a foothold in the contest before the break and although Benzema, Bale and Toni Kroos all failed to trouble Suntae, Santiago Solari's side eventually went in front a minute before the break.
Having sealed their place in a fourth final, Madrid can move past fierce rivals Barcelona with four titles should they win go on to lift the trophy, which would be a welcome boost in a stuttering season.
Bale had missed the 1-0 victory over Rayo Vallecano last weekend with an ankle problem but he was in irresistible form, albeit against limited opposition. "I can not worry that there is speculation about Madrid", Solari said.
The Spain worldwide is under contract at Real Madrid until 2022, but for the first time is seriously considering leaving after falling out of favour with coach Santiago Solari.
Over the past few months, there were reports linking Mourinho with a return to Serie A giants Inter Milan, who have struggled in recent seasons.
Kashima Antlers entered in the second round of the competition and edged Concacaf Champions League victor Chivas Guadalajara 3-2 to reach the semifinals.
"We see the final as a reward for this team that has been together a long time and given great joy to Madrid fans", Solari said.
Kashima did start brightly, Serginho finding room in the Madrid defence to get in a second-minute shot that was tipped wide by Thibaut Courtois, with Gen Shoji glancing a free header wide from the resulting corner.
Madrid's front two were tying Antlers in knots and two minutes before half-time, Bale struck.
Antlers midfielder Shoma Doi reduced the deficit with a consolation 12 minutes from time.
Toni Kroos, Sergio Ramos and Benzema all threatened Kwoun Sun-tae's goal before Bale brought some quality to proceedings with a well-placed finish following a neat one-two with Marcelo on the edge of the Kashima penalty area.
Bale scored twice in as many minutes as first, he capitalised on a poor backpass from Shuto Yamamoto and then drove into the far corner after Marcelo had, again, teed him up. Initially ruled offside, VAR showed Endo was level but a comeback never looked likely.
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