Damian Warner reclaimed the silver medal position and closed the gap between he and defending champion Ashton Eaton with a field-best time of 13.58s in the 110m hurdles.
Eaton, running alongside Warner in Heat 4, finished just behind the Canadian with a time of 13.80s.
Warner and Eaton earned 1,029 and 1,000 points for their respective performances. Eaton entered the event with a 132-point cushion over Warner, and therefore still maintains a very healthy lead in the overall competition standings.
Germany’s Kai Kazmirek, who ended the first day in second place, finished last in Heat 4 and falls to fourth overall.
American Jeremy Taiwo also dropped a spot, finishing third in Heat 3.
The athletes will start the seventh event of the decathlon, the discus throw, momentarily. Warner will hope to further close the gap on Ashton Eaton in what is decisively Eaton's weakest event. He finished 22nd among the field in London.
Overall top 10 as of 110m hurdles:
Ashton Eaton is in prime position to become the first man to successfully defend an Olympic decathlon gold medal since Bob Mathias in 1952.
The reigning "Most Athletic Man in the World" responded from a lackluster performance in the high jump with a superb run in the 400-meter dash, expanding his overall decathlon lead to 121 points at the halfway mark of the competition.
Eaton's time of 46.07s was the best of the night by over a half-second, topping his 400m time of four years ago – also best of the competition – by nearly a full second.
Germany's Kai Kazmirek posted the second-best time of the field, shooting him into second position overall with an even 4,500 points.
Canada's Damian Warner placed third, but drops a spot in the overall standings to third place.
Jeremy Taiwo of Team USA drops as well, finishing over 2.5-seconds behind Eaton. Taiwo had climbed as high as the third overall spot after winning the high jump event.
Overall top 10 after day 1:
A massive jump from Jeremy Taiwo catapulted the Seattle native from ninth place to within a point of second place overall in the men's decathlon.
On his third and final attempt, Taiwo lept 2.19 meters, greater than the height of NBA legend Shaquille O'Neal. Taiwo's jump was the highest out of all the decathlon competitors by three centimeters.
At 2.01m, Ashton Eaton finished the high jump tied for 14th, a relatively poor performance considering that Eaton tied for second in the event four years ago in London. The defending Olympic champion still holds the top spot in the overall standings, but his lead over Canada's Damian Warner shrinks from 95 to 68 points.
Overall top 10 as of high jump:
The 100-meter dash, long jump, and shot put events of the men’s decathlon took place earlier this morning. The high jump event is currently underway and 400-meter dash will follow at 8:20pm ET this evening to round out the first half of competition.
London 2012 gold medalist Ashton Eaton of Team USA sits atop the standings with 2,803 points, 95 points ahead of Canada’s Damian Warner. Kevin Mayer of France sits just seven points behind Warner with 2,699, and Brazil’s Luiz Alberto de Araujo rounds out the top four with 2,648 points after three events.
Four years ago, Eaton dominated the field on the first day of competition, winning three of five events – the 100m, long jump, and 400m.
He got off to another strong start in Rio this morning, winning the long jump by a massive 25 centimeters and placing second in the 100m, .16 seconds behind Warner.
Eaton placed 10th in the shot put, one of his weaker events. Mayer had the farthest throw of the day at 15.79m.
Other Team USA members Jeremy Taiwo and Zachery Zeimek sit ninth and tenth respectively in the overall standings. Zeimek finished third behind Eaton in the 100m, and Taiwo’s sixth-place, 14.92m shot put throw was tops among the Americans.
The decathlon continues at 8:20pm ET with the 400-meter dash.
Overall top 10 as of shot put: