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Two level quantum mechanical systems like spin 1/2 particles lend themselves as a natural qubit implementation. However, encoding a single qubit in several spins reduces the resources necessary for qubit control and can protect from decoherence channels. While several varieties of such encoded spin qubits have been implemented, accurate control remains challenging, and leakage out of the subspace of valid qubit states is a potential issue. Here, we realize high-fidelity single qubit operations for a qubit encoded in two electron spins in GaAs quantum dots by iterative tuning of the all-electrical control pulses. Using randomized benchmarking, we find an average gate fidelity of $\mathcal{F} = (98.5 \pm 0.1)\,\%$ and determine the leakage rate between the computational subspace and other states to $\mathcal{L} = (0.4\pm0.1)\,\%$. These results also demonstrate that high fidelity gates can be realized even in the presence of nuclear spins as in III-V semiconductors., Comment: Pascal Cerfontaine and Tim Botzem contributed equally to this work