Which USWNT players are on the brink of missing the Olympics?

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When the U.S. women's national team faces off against South Africa on Saturday (1 p.m. ET, on FS1), there will be little on the line for the team. After all, it's just a friendly and South Africa aren't very good.

But for the players trying to get a spot on the roster for the Olympics this summer, the stakes couldn't be higher. It will probably be their last chance to prove they should be going to the Olympic Games in Rio de Janeiro.

USWNT coach Jill Ellis has just 18 roster spots up for grabs, the smallest roster of any competition the USWNT participates in. That means that there are more players on the bubble than usual and Ellis has some very difficult decisions ahead.

For the players looking to break into the team, Saturday's match is crucial as the roster for Rio should be announced by their July 22 friendly. Here's a look at the players who need to make the most of Saturday to earn a spot on the Olympic roster:

Allie Long

She is the late bloomer of the group, but Long has her first real shot at representing the U.S. in an international tournament. She went about pushing for a roster spot the old-fashioned way: She worked really hard in club play with the Portland Thorns and kept getting better. At 28, she is best she has ever been.

But as fans and analysts beg Ellis to call up more players from the National Women's Soccer League, Ellis has always consistently cautioned that the national team environment is a much higher and difficult level. Exceling in the NWSL won't translate with the USWNT, but Long showed she may be one of the rarer exceptions during friendlies over the past few months. Of course, the USWNT has played some soft opponents in that time and Long's rise through the USWNT is recent enough that a spot in Rio far from a sure thing. It can go either way for Long and she'll be keen to prove she belongs on the USWNT.

Ashlyn Harris

First things first: No one is taking over Hope Solo's spot as the USWNT's goalkeeper. Solo's form is so dominant and has been for so long, that Harris is merely left to vie for a spot as her back-up. Her main competition is Alyssa Naeher, who it seems has quietly worked her way up into the USWNT's No. 2 goalkeeper spot. The interesting thing is, there's a not a huge difference between Naeher and Harris. While Harris has eight international caps, Naeher has just six. And while both are mostly solid, both can make costly mistakes at similar rates.

Their national team playing time is so limited, that we have to turn to the NWSL. Even there, the gulf between the two isn't very big. Harris has made 41 saves in 10 games for the Orlando Pride this season, allowing 9 goals, for a goals-against average of 0.90. Naeher has played one less match for the Chicago Red Stars and has made 30 saves, with 7 conceded, for a goals-against average of 0.78. It could be argued that Chicago has a slightly better defense, which gives Harris the edge. So, it seems there may not be much Harris can do -- but since neither goalkeeper is likely to see any minutes in pre-Olympic friendlies, Harris will need to make her case in USWNT training.

Heather O'Reilly

The right winger has been a workhorse and stalwart for the USWNT over the years. But for whatever reason, Ellis doesn't seem to rate O'Reilly very well. O'Reilly's style is one built less on technical precision and more on being a physical presence with a high work rate. It may not be very unpredictable -- everyone knows her favorite move is a long touch and sprint around a defender to the end line before a cross -- but it can be effective.

Still, since Ellis took over as coach in 2014, O'Reilly's playing time as diminished, with the more technical Tobin Heath often appearing on the right flank -- and even natural center forwards like Christen Press taking over the role. O'Reilly played just nine minutes during the World Cup last summer, and the team's depth has gotten even stronger since then. She will have to really impress Ellis and show how her style of play can fit into the team if she wants a shot at the Rio bench.

Megan Rapinoe

It's unlikely that Rapinoe will actually play on Saturday. In fact, it seems unlikely she will play any game minutes at all before the Olympics start in Rio. The left winger tore her ACL in December, which is a long and difficult injury to come back from. Being ready in time for the Olympics in August pushes right up against what would be a normal-but-good recovery time.

But if Ellis can bring Rapinoe to the Olympics, even at less than 100 percent, she surely will. Rapinoe is one of the most creative wingers in the world and her breezy, jokey personality has been a constant presence on the team throughout the years that surely keeps the team chemistry strong. Though Rapinoe won't play Saturday, Ellis will surely be looking for the 31-year-old to near the end of her recovery and show in training that she is close enough to being game-ready for Rio.

Whitney Engen

She was once looking like the next lock for a spot in the USWNT's central defense, but Engen has slowly been pushed out by roster competition. The rise of Julie Johnston, it seems, has been Engen's loss. Engen had the distinction of being the only outfield player to not see a single minute during the World Cup last summer. What she does have in her favor, however, is a lack of depth in the central defense.

Ellis' comments since the World Cup ended have suggested she is not looking only at Rio, but beyond to the 2019 World Cup in France. For that reason, 22-year-old Emily Sonnett looks like a much more appealing choice for the bench than 28-year-old Engen. Sonnett can be a key piece for years to come and major tournament experience in Rio would only help her development. Sonnett has also proven herself with the Portland Thorns as a steady presence, and that puts the pressure on Engen to show that she deserves that spot on the Rio roster.