RIO DE JANEIRO — The Aquatics Stadium nearly saw its first Katie Ledecky world record Sunday afternoon, as Ledecky finished one stroke away from resetting her own mark in the 400-meter freestyle. Her return to the pool for Sunday night’s final isn’t the only thing to watch, however, as a packed slate awaits.

Five takeaways from Sunday’s preliminary heats:

1. Katie Ledecky’s sweet spot: Though it was fun to see Ledecky sprint twice on Saturday and help the U.S. women to silver in the 4×100 freestyle relay, it’s even more enjoyable to watch her swim her longer distance events. She nearly lowered her own world record in the 400 free Sunday morning, and after calling her final stroke “lackadaisical,” she vowed that Sunday night’s final would go better. In other words, that world record might get reset during primetime, and Ledecky is about to secure her first gold medal of the Games.

2. Yulia Efimova competes: The reinstated Russian swimmer — twice suspended for banned substances — was quietly added to the heat sheet on Saturday, with no official explanation from FINA as to the reason for her reinstatement and eligibility for these Olympics. Efimova won her preliminary heat — there were audible boos from the crowd after she touched the wall — and ultimately qualified second heading into the women’s 100 breaststroke semifinals Sunday night behind American Lilly King. Efimova barely spoke to reporters following her prelim swim, saying only that she’s happy to be here.

3. No Phelps, no problem: The U.S. men’s 4×100 free relay did not qualify for the final a year ago at the 2015 world championships in Kazan (an event in which 22-time Olympic medalist Michael Phelps was ineligible to compete), a major embarrassment for all of American swimming. Now, the U.S. men can’t afford a similar slip-up. So far, so good, as the foursome of Jimmy Feigen, Ryan Held, Blake Pieroni and Anthony Ervin got the job done quite well in the morning prelims. The Americans qualified second, behind Russia and ahead of Australia, for Sunday night’s final. If this were up to us, we’d put Ervin on the relay tonight with Caeleb Dressel, Phelps and Nathan Adrian — giving the U.S. its best shot at redemption. As for 2015 worlds, Ervin said Sunday, “That’s ancient history, man.”

4. Kathleen Baker blitzes through prelims: The 19-year-old swam her best time in the women’s 100 backstroke at an opportune time, setting her up as the surprising top qualifier for Sunday’s semifinals ahead of Australian Emily Seebohm, who owns the Olympic record in this event. American Olivia Smoliga qualified fifth, just behind Hungary’s Iron Lady, Katinka Hosszu, who won her first Olympic gold medal on Saturday in record-breaking fashion in the 400 IM.

5. Conor Dwyer’s comeback: The top qualifier heading into the men’s 400-meter freestyle final Saturday, Dwyer ultimately missed the medal stand, finishing a disappointing fourth in an event in which he’d been in position to medal until the final 50 meters. A day later, Dwyer seemed to forget the frustration of that swim and focus on what’s next: The 200 free. Dwyer won his prelim heat with a solid time of 1:45.95, good for fourth heading into semis and just 0.2 seconds off that of top qualifier Sun Yang.