Arsenal, Chelsea, Frankfurt and Valencia have made it to the last four; they will battle for the final spots.
Arsenal vs Valencia
Both teams have their problems but should relish a return to a competition that has brought the best out of them.
Kick Off: 21:00 CET, Thursday
Probable Lineups
After three straight Premier League defeats, Arsenal will be glad to find themselves in the relatively comforting surrounds of the UEFA Europa League again, having sparkled against Napoli in the last round.
Team news
Arsenal look set to be able to welcome back both Mesut Ozil and Nacho Monreal from injury for the game. However, Aaron Ramsey and Dennis Suarez will be missing for the Gunners.
Valencia are without midfielder Geoffrey Kondogbia, winger Denis Cheryshev and full-back Cristiano Piccini.
Key battle
Ainsley Maitland-Niles vs Gonçalo Guedes: Guedes carries much of Valencia's attacking threat. This time last year he was running on empty, but now it's quite the opposite now.
Fit, full of power and chutzpah, he's beguiling to watch. Not only super-quick down the left wing, he now drifts into inside-left and second striker positions. Hard to handle, and the attack-minded Maitland-Niles will have to be on his guard.
Form guide
Valencia are unbeaten since dropping down from the Champions League to the Europa League (W5, D1) but have lost two games in a row heading into Thursday.
Arsenal are in even worse form, losing three on the spin since winning 1-0 away to Napoli in their Europa League semi-final second leg. This tie could go either way.
Did you know?
Arsenal defender Shkodran Mustafi was a Valencia player from 2014 to 2016, making 81 outings.
There are two former Gunners in the Spanish side's ranks: Francis Coquelin made 160 appearances for the north London club between 2008 and 2018, while Gabriel was an Arsenal player between January 2015 and August 2017, playing 64 games. Then there's Unai Emery, who was Valencia coach for four seasons until 2012.
Eintracht Frankfurt vs Chelsea
Chelsea face a difficult away trip when they travel to the Commerzbank-Arena to take on Eintracht Frankfurt in Thursday's Europa League semi-final first leg.
Kick Off: 21:00 CET, Thursday
Probable Lineups
Focus
Chelsea head into their 59th game of the season having maintained two chances to achieve Champions League football, but Maurizio Sarri knows that success or failure over the next four games could decide whether he remains at Stamford Bridge next season.
There have been signs over the past two months that Chelsea's players are becoming more accustomed to their head coach's favoured style of play, while Sarri has found himself having to tinker with his system to successfully accommodate the likes of Ruben Loftus-Cheek and Callum Hudson-Odoi.
Team news
Ante Rebic is suspended, while Mijat Gacinovic is doubtful and Sebastien Haller, Marc Stendera, Timothy Chandler and Taleb Tawatha are all injury absentees.
Antonio Rudiger and Willian both went off injured in Sunday's draw at Manchester United, with the former now unlikely to feature here and the latter a doubt.
Key battle
Danny da Costa vs Eden Hazard: Rampaging down the right, the Frankfurt wing-back has been a revelation this season, chipping in with crucial goals in the Bundesliga and the UEFA Europa League, but an in-form Hazard will likely force Da Costa to spend more time on the back rather than the front foot.
If Da Costa manages to nullify Chelsea's chief attacking threat – easier said than done – then the Eagles stand a chance of achieving a famous result. Present Hazard an opportunity, though, and he usually snaffles it up – and the London club usually prevail.
Form guide
Did you know?
This tie pits together the highest-scoring sides in this season's competition, 30-goal Chelsea against 28-goal Frankfurt.