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Recently developed adaptive modulation schemes for opportunistic decode-and-forward (DF) relaying require knowledge of instantaneous channel-state-information (CSI) of all links. This paper aims at reducing signalling overhead of modulation-adaptive opportunistic DF relaying through exploitation of mixed channel knowledge. A novel semi-deterministic approach is proposed for performing joint rate-adaptation and best-relay selection. The selection process employs two criteria using mixed channel knowledge. A new metric namely the probability of missing the best relay (PMBR) is introduced, as only when the best relay is selected, the full diversity order can be achieved. It is shown that the proposed approach can achieve a good trade-off between the spectral efficiency and signalling overhead.