Rugby’s return to the Olympic Games after an 88-year absence has already provided plenty to justify its inclusion in Rio in 2016. In the women’s event, a thrilling Australia side claimed gold with a win over old rivals New Zealand. But if the competition largely went to form for the women, the unpredictable nature of the seven’s form of the game was laid bare on the opening day of the men’s event.

Both New Zealand and Australia, two rugby heavyweights who faced off in the final of 15-a-side World Cup last year, suffered not only surprise losses but their key players to tournament-ending injuries. The battle for medals has been blown wide open and the United States is still very much in the hunt.

While the Americans may not exactly be an established power in rugby, the country is the defending gold medalist in the sport, having claimed victory all the way back in Paris in 1924 when rugby was played in the full 15s version. And the U.S. has been experiencing a rugby boom in the past few years, particularly in sevens, a much quicker, more explosive version played over two halves of seven minutes rather than 40 minutes as in regular Rugby Union. Last year, the U.S. won its first ever Sevens World Series tournament title.

In Carlin Isles, the U.S. has one of the most thrilling players in the sport and one whose bursts of speed made him a YouTube sensation, despite having only taken up rugby a few months earlier after failing to get on the sprinting team for the 2012 Olympics. But the team also has another man generating headlines. On Tuesday, New England Patriots safety Nate Ebner became the first active NFL player to compete at the Summer Olympics.

While he may be a Super Bowl champion, Ebner played the rugby as a junior and he was given permission by Patriots coach Bill Belichick to miss training camp and try to bring a medal home.

Nate Ebner Nate Ebner runs through an opponent to score a try in the United States' win over Brazil. Photo: Getty Images

His first Olympics appearance was a painful disappointment both individually and for his team as Argentina scored a try with the last play of the game to steal victory. But the U.S. struck back in its second game of the day, with both Ebner and Isles scoring tries in a 26-0 win over Brazil that kept the Americans’ hopes alive of making the quarterfinals.

“Nate’s a phenomenal athlete, and he’s got a rugby background growing up,” U.S. captain Madison Hughes said after the win over Brazil. “He adds a bit of depth to our team. He’s got pace, he’s got physicality, and he’s got a rugby brain.”

The U.S. currently sits third in Pool A, with the top two teams from each of the three four-team groups, as well as the two best third-placed teams, going onto the quarterfinals, which take place on Wednesday evening. Ebner, Isles and co face a tough task on Wednesday when it takes on Fiji, a real specialist in the event and unbeaten through two games, in its final group match.

Other matches to look out for on Wednesday afternoon are Australia, beaten heavily by France and now without key man Lewis Holland to injury, playing for its Olympic life against an unbeaten South Africa. New Zealand will be in the same position, too, when taking on Great Britain. The All Blacks suffered the dual blow of being upset by Japan on Tuesday and losing Rugby World Cup winner Sonny Bill Williams to an Achilles rupture. Another player, Joe Webber, was also ruled out of the competition.

“It’s devastating for Sonny to lose him so early in the tournament, and Joe Webber, both influential players as well,” New Zealand coach Gordon Tietjens told reporters at Deodoro Stadium in Rio. He loves the game of sevens and I’ll be surprised if he doesn’t play sevens again.

“Sonny Bill’s a great professional, he’s very infectious, he certainly helps the younger players and has a great work ethic. It’s such a shame because he’s got to understand the game of sevens so well. He’s absolutely shattered.”

Olympic Men’s Rugby Sevens ScheduleWednesday

10 a.m. France vs. Spain

10.30 a.m.: South Africa vs. Australia

11 a.m.: Kenya vs. Japan

11.30 a.m.: New Zealand vs. Great Britain

12 p.m.: Argentina vs. Brazil

12.30 p.m.: Fiji vs. United States (TV: USA network)

3 p.m.: Placing 9-12 match

3.30 p.m. Placing 9-12 match

4 p.m.: Quarterfinal 1

4.30 p.m.: Quarterfinal 2

5 p.m.: Quarterfinal 3

5.30 p.m. Quarterfinal 4

Thursday

11.30 a.m.: Placing 11-12 match

12 p.m.:  Placing 9-10 match

12.30 p.m.: Placing 5-8 match

1 p.m.: Placing 5-8 match

1.30 p.m.: Semifinal 1 (TV: NBCSN)

2 p.m.: Semifinal 2 (TV: NBCSN)

4.30 p.m.: Placing 7-8 match

5 p.m.: Placing 5-6 match

5.30 p.m.: Bronze medal match (TV: NBCSN)

6 p.m.: Gold medal match (TV: NBCSN)

Live Stream: All matches will be streamed on NBCOlympics.com.