Phelps and Lochte, owners of a combined 33 Olympic medals, are in the 200m individual medley final.
They are well-versed in the event with Phelps owning the last three Olympic 200m IM titles (sharing the podium with Lochte each time) and Lochte owning the last four World 200m IM titles.
Already in Austin, Phelps won the 100m butterfly on Friday and Lochte captured the 400m IM on Saturday.
Phelps has not bettered Lochte in a 200m IM since the 2012 Olympics.
Also Sunday, four-time 2012 Olympic champion Missy Franklin goes in the 100m backstroke final, searching for her first win of the meet.
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