The first of tonight's swimming action resulted in the United States' Cullen Jones and Brazil's Cesar Cielo finishing joint top in the first heat of the 50m freestyle semi-final.
The pair reached the end of the pool in 21.54s, while the second heat saw another Brazilian, Bruno Fratus, reach the quickest time 21.63s.
Anton Ervin for the USA clinched a time of 21.62s in the first heat with Trinidad and Tobego's George Bovell recording 21.77s.
Meanwhile, American hopeful Ryan Lochte is on course to claim a second gold medal in the 200m backstroke final.
