Michael Phelps pips Serbian rival Milorad Cavic to qualify second fastest for the 100m butterfly final.
The duo' s rivalry extends back to the Beijing games where it looked like Cavic was going to beat Phelps, but the American famously took gold in a fingernail finish.
A quick finish from Phelps propels him from last place to first in the heats 50m of the men's 100m butterfly.
Their rivalry will resume once again in Friday night's final.
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