SPOILER ALERT: 4 to Watch: Simone Biles Competes on Balance Beam

On Day 10 of the Olympics competition, Simone Biles competed on the balance beam; Allyson Felix will race for her first Olympic 400-meter gold; a heavyweight champion will be crowned, and the U.S. men face Mexico in volleyball.

Here are four of the biggest Things to Watch during the Summer Games Monday, and there is a gymanstics spoiler.

1) Simone Competes on Balance Beam

On Monday, Simone Biles competed on the balance beam, looking for her fourth gold medal, but had to settle for bronze after almost falling.

No American gymnast has won four gold medals in one Olympics since 1904, when Anton Heida won five.

Biles won her third gold Sunday, this time on the women's vault, landing both her vaults to near perfection to win the individual apparatus championship. 

Biles, who had the highest score in qualifying, soared over the competition once again. Landing an Amanar with a small hop on the landing to score a 15.9, and a Cheng on the second attempt that scored 16.033, there was no doubt.

How to Watch: Watch the event during NBC4's prime-time coverage beginning at 8 p.m. 

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Gold medalist Simone Biles of USA poses during the medal ceremony for the Women's Individual All-Around Final on day 6 of the Rio 2016 Olympic Games at Rio Olympic Arena on Aug. 11, 2016, in Rio de Janeiro, Brazil.

2) Felix Races for Gold

Allyson Felix will race for her first Olympic 400 meter gold medal.

Felix made her Olympic debut at the 2004 Athens Games at age 18, claiming the 200-meter silver behind Jamaica's Veronica Campbell-Brown. At the 2008 Beijing games, Felix again finished second to Campbell-Brown. Then at the 2012 London Games, Felix finally broke through, winning her first Olympic 200-meter gold medal.

Felix also earned gold medals in the 4x100 and 4x400 relays in London to become the first female U.S. track athlete to win three gold medals at a single Olympics since Florence Griffith-Joyner in 1988.

How to Watch: See Felix run for gold live online when coverage of the event begins at 7:20 p.m. ET, or see it on NBC4's prime-time coverage.

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Allyson Felix of the United States competes in round one of the Women's 400m on Day 8 of the Rio 2016 Olympic Games at the Olympic Stadium on Aug. 13, 2016, in Rio de Janeiro, Brazil.

3) A Heavyweight Champion Will Be Crowned

A men's heavyweight boxer will win a gold medal. Russia's Evgeny Tishchenko faces Kazakhstan's Vassiliy Levit. No Americans reached the final in the heavyweight class, but Team USA did compete Monday. 

Women's lightweight Mikaela Mayer boxed in a quarterfinal and lost to Russia's Anastasiia Beliakova. Men's flyweight Antonio Vargas lost in a unanimous decision to Uzbekistan's Shakhoibidin Zoirov in the round of 16.

The men's boxing delegation is the smallest at a non-boycotted Olympics since 1908.

How to Watch: Mayer fights at 4 p.m., and the heavyweights fight for gold at 6:15 p.m. All matches are viewable online (watch Mayer here, and see the heavyweight final here). The heavyweight final will be broadcast on NBC Sports Network during the evening session (9:30 p.m.-midnight).

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USA's Shakur Stevenson, right, fights Brazil's Robenilson De Jesus during the Men's Bantam (56kg) match at the Rio 2016 Olympic Games at the Riocentro - Pavilion 6 in Rio de Janeiro on Aug. 14, 2016.

4) US Men Beat Mexico, Advance to Volleyball Quarterfinals

The U.S. men's volleyball team -- with locals Aaron Russell and Reid Priddy -- defeated Mexico in three sets Monday to clinch a spot in the quarterfinals.

Team USA won 25-23, 25-11, 25-19 and will next play in the knockout round on Wednesday.

This young group, with eight first-time Olympians and a trio of veterans leading the charge, has shined during this four-year cycle leading up to Rio when under the most pressure.

"This tournament is an emotional rollercoaster ride," coach John Speraw said.

United States' Micah Christenson prepares to serve during a men's preliminary volleyball match against Canada at the 2016 Summer Olympics in Rio de Janeiro, Brazil, Aug. 7, 2016.
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