The 2010 Commonwealth Games medallist made a safe progression to the final of the men’s 400m freestyle with a time of 3:50.66.
Nick Grainger was the fastest qualifier in the heats with a time of 3:50.24 – slicing nearly five seconds off his lifetime best in the process – while Daniel Fogg will also provide competition for defending champion Renwick.
But with places to Barcelona at the end of July up for grabs, Renwick knows securing an ‘A’ standard time is a must.
He said: “It was a quick heat time, it’s always good to get the first swim out of the way because you always feel a bit rusty.
“Hopefully I can get a good recovery and go a few seconds faster tonight a get that qualifying time.
“It would be nice to win the race tonight, it’s going to be a good race, head-to-head, but everyone can step up a bit more and I’m aware of that.”
Elsewhere, Jazz Carlin booked the best lane in Thursday’s 800m women’s final with a heats time of 8:30.06, and she was delighted with her early form.
“It was my first swim and it was really about getting into the mix," said Carlin, who missed last year's Olympics after illness and injury hit her preparations.
"You are always most anxious about the first swim but I was really pleased because that was the fastest I have ever swam in the morning.
“I am really excited for the final and I was not far off the Worlds qualifying time in the heats so hopefully I can make that in the final.”
Meanwhile, Chris Walker-Hebborn was the fastest qualifier in the men’s 100m backstroke ahead of favourite Liam Tancock, while Francesca Halsall qualified fastest in the women’s 50m freestyle heats in 25.06 seconds.