World champion Cseh led from start to finish to beat Denmark's Viktor Bromer by 2.44 seconds, with Tamas Kenderesi taking third for Hungary on a great night for the central Europeans in the same London pool where they endured a disappointing 2012 Olympics.
Asked about Phelps's comeback from retirement, Cseh told Reuters: "It is nothing special, I know maybe he will be in the Olympics, so I need to beat him and the others since I want to be the best. This is my main motivation, my main goal, to win a gold medal in the Olympics."
That is something that has eluded the 30-year-old throughout a long career which has brought him two world titles, spaced 10 years apart in 2005 and 2015.
He has often been the 'nearly man' behind Phelps, the most successful Olympian of all time, who retired after the London Games but reversed his decision two years later.
Cseh took Olympic silver behind Phelps in the 200m butterly and the 200m and 400m individual medleys in Beijing in 2008, and bronze behind Phelps and Ryan Lochte in the 200m medley in 2012. Rio represents almost certainly his last Olympic chance.
In other action, Cseh's compatriot Katinka Hosszu beat Britain's Siobhan-Marie O'Connor and Hannah Miley to the gold in the women's 200m individual medley, having grabbed the 400m medley and 200m backstroke titles earlier in the week.
