Weekends are times when some people struggle for motivation. Caeleb Dressel apparently has no such issues.
Dressel smashed the previous 100-meter butterfly world record of 48.08 seconds to 47.78, pushed by previous holder Chad Le Clos, in turn becoming the first man ever to complete the 100-meter butterfly in under 48 seconds.
Then, around 40 minutes later, he returned to the pool to reduce his own 50-meter freestyle world record time by 0.08 from 20.24 seconds to 20.16.
"It was a really exciting day, and I'm so pleased with the times," the 24-year-old told BBC Sport afterwards. "It's like an amusement park for swimmers, and I'm loving it at the ISL this season, but we want to win this as a team -- that's the end goal."
Dressel's sublime speed helped his team, Cali Condors, to a first-day lead over defending champions Energy Standard, with London Roar third and LA Current fourth.
The ISL is contested in a shorter, 25-meter pool. Bigger events, such as the Olympics, world championships and national championships, are held in 50-meter pools.
Dressel's double world record-breaking day comes just a weekend after he became the first swimmer ever to dip under 50 seconds in the 100-meter individual medley.
He took almost half a second off the previous record set by the Russian Vladimir Morozov in 2018, finishing with a time of 49.88.
Afterwards, three-time Olympic gold medal winning swimmer and now ISL commentator Rowdy Gaines called Dressel: "The best swimmer on the planet, bar none."
"He is definitely not human," Gaines said.
Dressel burst onto the scene in the 2016 Rio Olympics, winning two gold medals in relays. Since then, he has gone on to win 13 golds medals and two silvers at the last two World Championships.
He is expected to be one of the top stars at next summer's Tokyo Olympics and could qualify for seven separate events including relays.