Michael Phelps, left, along with teammate Caeleb Dressel, was reason to be pumped. (Dave Hunt/European Pressphoto Agency)

Day 2 in Rio featured an abundance of exciting moments for U.S. fans, but it saved its best for last. In Sunday’s final event, Michael Phelps, a late confirmation for the men’s 4×100-meter freestyle relay team, showed vintage form in helping secure a somewhat unlikely gold medal for the Americans, boosting his own record totals to 19 golds and 23 medals overall in the process.

Phelps wasn’t as singularly dominant as Katie Ledecky was, but his dramatic appearance and debut performance in his fifth Games made for quite the crowd-pleasing highlight. For her part, Ledecky proved once again that she is often essentially swimming against herself. She took a massive early lead in the women’s 400-meter freestyle and cruised to a gold medal, with the only drama being whether and by how much she would break her own world record in the event.

As it turned out, Ledecky broke her two-year-old mark by nearly two seconds (3:56.46); her closest competitor Sunday, silver medalist Jazz Carlin of Australia, finished over five seconds behind. Another U.S. swimmer, Leah Smith, took the bronze, and fellow American — and “Super Mom” — Dana Vollmer did the same in the women’s 100-meter butterfly.

Other world records that fell at the Olympic Aquatics Stadium on Sunday included the winner of Vollmer’s event, Sweden’s Sarah Sjostrom, lowering that mark to 55.48, and Britain’s Adam Peaty notching an unprecedented 57.13 while winning gold in the men’s 100-meter breaststroke. But the pool wasn’t the only place where Day 2 events went swimmingly for the U.S.

Fierce Five 2.0, a.k.a. the American women’s gymnastics team, was so much better than its competition in preliminary events that The Post’s Liz Clarke called it “the most prohibitive Olympic gold medal favorite since USA Basketball’s Dream Team of 1992.” With three-time world all-around champion Simone Biles leading the way, the U.S. emerged with a total score of 185.238 points, with the next two nations, China and Russia, posting 175.279 and 174.620, respectively.

The only down note for the U.S. came in the all-around, and it was solely because of an Olympic rule that any one country can send a maximum of two performers to the final. That meant that defending champion Gabby Douglas will be left out, even though she finished third overall Sunday, behind teammates Biles and Aly Raisman.

Elsewhere, Alexander Massialas gave the U.S. its first medal in individual men’s foil fencing since 1960, when he took silver. American Corey Cogdell took bronze in women’s trap shooting, while her husband, Bears defensive lineman Mitch Unrein, cheered her on from Chicago’s training camp.

A disappointment for the U.S. came in the 141-kilometer-long women’s road race, where Mara Abbott was overtaken in the final 150 meters by the three women who would celebrate on the podium. That wasn’t nearly as painful an experience, though, as that of the Netherlands’ Anna van Vleuten, who was leading the race before suffering a horrific crash which left her with spinal fractures and a concussion.

The top-seeded doubles team of Venus and Serena Williams took its first-ever loss in Olympic play, although the latter won her singles match, salvaging something from a poor showing thus far by American tennis players. The U.S. men’s team volleyball squad got off to a rough start, dropping its first match to lower-seeded Canada in three sets.

Memorable moments for other countries included Kosovo’s first medal, and a gold at that, won by Majlinda Kelmendi in the women’s under-52-kilogram weight category. China’s Wu Minxia, 30, became the first women’s diver to win a gold medal in four consecutive Games and the oldest woman to win an Olympic diving gold medal.

Chnia also celebrated a dramatic gold medal for Long Qingquan, the 56kg weightlifter and 2008 champion who set a world record with his final effort, in the process edging North Korea’s Om Yun-Chol, the 2012 gold medalist. A head-to-head matchup between Novak Djokovic and Juan Martin del Potro saw the world’s No. 1 player get ousted, this after del Potro spent 40 minutes stuck in an elevator before being rescued by members of the Argentinean handball team.

Day 3 brings the debut of fencer Ibtihaj Muhammad, the first U.S. athlete to compete in the Games while wearing a hijab, and whose teammate, Mariel Zagunis, begins her quest for a third gold. Phelps will swim in his first individual event, the 200-meter butterfly prelims, while in another pool, the U.S. men’s water polo team takes on Spain.

Monday also brings the first action for Marti Malloy, who won a judo bronze for the U.S. in 2012, and a shot at redemption for teammate Nick Delpopolo, who was kicked out of the London Games after testing positive for marijuana. Of course, there’s much more than that on top, so stay tuned — or check this space tomorrow to see what you might have missed.