To prevail on a claim of ineffective assistance of counsel, a defendant bears the burden of establishing that counsel's performance was deficient and that the deficient performance produced prejudice. See State v. Sanchez, 201 Wis. 2d 219, 232-36, 548 N.W.2d 69 (1996). To show prejudice, the defendant must demonstrate "that there is a reasonable probability that, but for counsel's unprofessional errors, the result of the proceeding would have been different. a reasonable probability is a probability sufficient to undermine confidence in the outcome." Strickland v. Washington, 466 U.S. 668, 694, 80 L. Ed. 2d 674, 104 S. Ct. 2052 (1984). Ineffective assistance of counsel claims present mixed questions of law and fact. See State v. Pitsch, 124 Wis. 2d 628, 633-34, 369 N.W.2d 711 (1985). A trial court's factual findings must be upheld unless they are clearly erroneous. See State v. Harvey, 139 Wis. 2d 353, 376, 407 N.W.2d 235 (1987). Whether counsel's performance was deficient and, if so, whether the deficient performance prejudiced the defendant are questions of law, which we review de novo. See Pitsch, 124 Wis. 2d at 634. The defendant has the burden of persuasion on both prongs of the test, and a reviewing court need not address both prongs if the defendant fails to make a sufficient showing on one. See Strickland, 466 U.S. at 687, 697.