The Champions League returns and with just four teams left, the path to glory is wide open. Any of the remaining sides has a chance but equally, question marks surrounding their challenge. Can Liverpool crack Lionel Messi & Co. to deny their Treble push? And which of the "underdogs" will prevail in the other semifinal?

Here's everything you need to know, from the superheroes and the sidekicks who can decide the ties to the keys for victory.

Jump to: Tottenham-Ajax | Liverpool-Barcelona

Tottenham vs. Ajax

WHEN: Tuesday, April 30, 3 p.m. ET / 7 p.m. BST

BACKSTORY: Ajax are doing their glorious past proud, having eliminated perennial Serie A champs Juventus and Champions League holders Real Madrid to get into the semifinals. The quality, freshness and fearlessness displayed in those matches have made them the neutrals' favorites. They are tied at the top of the Dutch league and in the Dutch Cup final, so they, too, have Treble aspirations.

Tottenham have lost more than half the games they have played since Valentine's Day, but they are still hanging on to third place in the Premier League and knocked out mighty Manchester City to get this far. That they're in the final four is remarkable given the rash of injuries they've suffered -- most notably to captain Harry Kane, who is out of both legs -- and their limited resources. Limited relative to most, but not Ajax: in any other season, Spurs would be the Cinderella team at this stage.

HOW THEY MATCH UP: Tottenham are at times perhaps more pragmatic, but there's a lot of common ground between the way these two sides play, and with the Spurs trio of Toby Alderweireld, Jan Vertonghen and Christian Eriksen having combined to spend nearly three decades at Ajax at youth and senior levels, there will be few surprises.

Given Kane's absence and the fact that Ajax's approach changes little home and away (regardless of the opposition's pedigree), you can imagine Spurs exploiting the counterattack, particularly with the fleet-footed Lucas Moura.

TOTTENHAM'S STAR TO WATCH: Eriksen. He's at a crossroads in many ways. He has yet to extend his contract, which expires in June 2020, suggesting he's open to a summer move. Wouldn't a decisive turn in the Champions League semifinal be the ultimate shop window? He's facing his past, but he's also the man who can create out of nothing for Tottenham and who may have to carry them, as he's done so many times before.

AJAX'S STAR TO WATCH: Frenkie De Jong. He epitomizes the modern two-way midfielder, with the added wrinkle that he carries himself with a charisma and a personality far beyond his years. He's making the jump to Barcelona in the summer but very few 21-year-olds have seemed this ready for that kind of quantum leap.

X-FACTOR, TOTTENHAM: Dele Alli. He hasn't put up the gaudy numbers of past seasons (he had 22 goals from midfield in 2016-17 compared to seven this year), partly because he missed seven weeks due to injury and partly because he has had to reinvent himself as a hole-plugging jack of all trades given Spurs' many injuries, particularly in the middle. But he has the dynamism and selflessness to match Ajax, and if he can keep Ajax from bossing the midfield, he can turn this Tottenham's way.

X-FACTOR, AJAX: David Neres. At 22, he's still very raw, but his dribbling ability and unpredictability make him a real handful. Within a philosophy that carefully balances the team versus the individual, he can lean toward the latter at times (which may be why he had a bumpy start to the campaign), but that makes him no less valuable over 180 minutes.

TOTTENHAM NEED TO... find a makeshift midfield combination that can see them through (which will not be easy, given the likely absences of Moussa Sissoko and Harry Winks) without leaving the initiative to Ajax, while getting Eriksen into positions where he can create.

AJAX NEED TO... not let the intensity and fearlessness drop simply because Tottenham's pedigree isn't equal to that of their previous Champions League opponents (Juventus, Real Madrid and Bayern), while ensuring the center-back duo of Daley Blind and Matthijs de Ligt need to do as little open-field defending against Lucas Moura as possible.

PREDICTION: Tottenham 2-1 Ajax. Momentum isn't with Tottenham and the midfield is a mystery but they otherwise don't match up badly against Ajax.

Barcelona vs. Liverpool

WHEN: Wednesday, May 1, 3 p.m. ET / 7 p.m. BST

BACKSTORY: Barcelona won La Liga on Saturday and will play Valencia in the Spanish Cup final later this month, which means they're still on track for the Treble. It would be their third since 2008, which is some achievement when you consider that it had only been achieved three times in the first 40 years of European football.

Liverpool reached the Champions League final last year, of course, but unlike then -- when they went out of domestic contention early -- they are now neck-and-neck with Manchester City for the Premier League title. They've won 10 games in a row in all competitions and look stronger and more complete than last year.

HOW THEY MATCH UP: It used to be a rarity for a team to score 100 goals in all competitions, but Liverpool passed that mark on Saturday and Barcelona did it back in February. Yet though both can (and do) score plenty, there's a more patient, defensive dimension to what they do as well. Jurgen Klopp only wheels out his "heavy metal" all-out press intermittently, and compared to prior incarnations, this Barca side are more safety-oriented in the middle of the park. That fact that both have outstanding goalkeepers in Marc-Andre ter Stegen and Alisson adds another layer of solidity.

BARCELONA'S STAR TO WATCH: Lionel Messi. He's averaging more than a goal a game (again) while playing an even bigger role than before in terms of dropping deep and creating chances for others. You can find your own superlatives here.

LIVERPOOL'S STAR TO WATCH: LIVERPOOL'S STAR TO WATCH: Virgil van Dijk. Few signings have been as transformative in recent years as the Dutchman's arrival at Anfield in January 2018. A colossus in the air, his work in reading the game and marshalling a back four in which both fullbacks are often attacking at the same time is second to none.

X-FACTOR, BARCELONA: Jordi Alba. He has 16 assists in all competitions this year from left back and a nearly supernatural understanding with Messi, who often cuts in from the opposite flank. He'll be up against Trent Alexander-Arnold and Mohamed Salah, which means Liverpool will need to get a midfielder helping out when he advances. Except they can't overcommit to him, because Messi is on the opposite side...

X-FACTOR, LIVERPOOL: Roberto Firmino. He's a center-forward who doesn't score that much (he has broken the 20-goal mark just twice in his career) but his prodigious movement and off-the-ball work rate more than make up for it. Between Sergio Busquets in midfield and Gerard Pique at the back, Barcelona won't want to lose sight of him.

BARCELONA NEED TO... Dictate the game in midfield and pin back Alexander-Arnold and Andrew Robertson, Liverpool's full-backs, while denying service to Salah and Sadio Mane, particularly in transition.

LIVERPOOL NEED TO... Be aware of Messi at all times, but particularly when he drops back to create. Ensure the front three don't get isolated, while also maintaining their shape.

PREDICTION: Liverpool 1-2 Barcelona. Two evenly matched sides but one of them has Messi, the other does not.