[en] The electroweak process e⁺e^- → μ⁺μ^- and e⁺e^- → τ⁺τ^- are studied in the energy range √s = 52.0 ∼ 61.4 GeV, using the TOPAZ detector at the TRISTAN e⁺e^- collider. From an integrated luminosity of L = 74 pb^-1, the lowest order cross sections and forward-backward asymmetries measured for these reactions are [σ_μμ] = 25.4 ± 0.9 ± 1.2 pb, [A_μμ] = 32.2 ± 3.1 ± 1.1% [σ_ττ] = 27.1 ± 1.1 ± 1.2 pb, [A_ττ] = -33.9 ± 4.9 ± 1.0%, at an average energy of (√s) = 57.87 GeV. At ∼10 standard deviations from QED, this represents the most significant measurement of the asymmetry for lepton pairs to the present date. Weak axial vector couplings of a_ea_μ = 0.96 ± 0.09 ± 0.01, and a_e a_τ = 1.01 ± 0.14 ± 0.01 are determined from the asymmetry. These results agree with the electroweak standard model prediction for γ-Z⁰ interference below the Z⁰ pole. Lepton compositeness is also tested. Lower limits in the range 2 ∼ 5 TeV at the 95% confidence level are placed on the compositeness scale parameters for μ and τ. No evidence for compositeness is observed