Pedro Rodriguez, Willian and Callum Hudson-Odoi scored a goal apiece for Chelsea, which was in full control throughout and easily could have translated its dominance into more goals. It was the third win in three games in all competitions for Chelsea as coach Maurizio Sarri has managed to turn things around after a string of poor results.
Fellow London club Arsenal, though, is in trouble following a 3-1 loss at Rennes. The French club rallied from an early deficit after Arsenal was reduced to 10 men to spoil goalkeeper Petr Cech's return to the city where he spent two seasons early in his career.
Arsenal took the lead through Alex Iwobi after just four minutes but the game turned when Sokratis Papastathopoulos was sent off late in the first half and Rennes equalized from the resulting free kick.
The 36-year-old Cech, who is retiring at the end of this season, made some great saves against the attack-minded home side but couldn't do anything to prevent his former club from building a massive advantage ahead of next week's return leg at the Emirates.
Former European Cup winner Benfica will also need a comeback after losing 1-0 at Dinamo Zagreb, and there was another surprise in Spain where Slavia Prague held five-time Europa League champion Sevilla to a 2-2 draw.
Napoli cruised past Salzburg 3-0 and Valencia beat Krasnodar 2-1, while high-scoring Eintracht Frankfurt was held to a 0-0 draw at home by Inter Milan. Also, Villarreal struck twice in the second half to earn a 3-1 away win at Zenit St. Petersburg.
The return games take place on March 14. Here's a look at Thursday's games:
CHELSEA IN CHARGE
Sarri made six changes from Sunday's 2-1 victory over Fulham, and two of those players made the difference in the first half.
Olivier Giroud, who scored six goals in the previous five Europa League games, set up Pedro with a brilliant back-heel pass in the 17th minute for a well-deserved lead.
Pedro squandered more chances as Dynamo had no answer to Chelsea's dominance.
Willian added the second in the second half, curling in a free kick from outside the area before Hudson-Odoi finished it off in the final minute.
It all started so well for Arsenal, but then it just went from bad to worse.
Just four minutes into the game, Iwobi danced around his marker on the left side of the box before curling the ball inside the far post of goalkeeper Tomas Koubek with what looked like an attempted cross.
Arsenal was reduced to 10 men four minutes before halftime after Sokratis picked his second yellow card, and Rennes immediately made the most of the subsequent free kick. Benjamin Bourigeaud saw his attempt blocked by the defensive wall before unleashing an unstoppable drive on the rebound.
After Nacho Monreal's own goal in the 65th, Ismaila Sarr completed a dominant performance by Rennes, heading home from close range two minutes from time.
SEVILLA SET BACK
Wissam Ben Yedder was on target even earlier than Iwobi, punishing Slavia Prague's poor defending within the opening minute to give Sevilla an early lead.
It was the seventh goal for the forward in the Europa League this season, joining Salzburg's Munas Dabbur atop the scoring table.
The hosts looked dominant but Miroslav Stoch equalized for Slavia with a deflected shot from the edge of the area 25 minutes later.
Sevilla quickly re-established the lead after just three minutes. Munir El Haddadi netted with a volley from a cornerk, his first goal since his transfer from Barcelona in January.
But the teams were level again before the interval with Alex Kral scoring from another corner, sending the ball into the net with his shoulder.
Napoli Coach Carlo Ancelotti kept a perfect record in the Europa League with a third win in his first three games, this time at the expense of last year's semifinalist Salzburg.
Forward Arkadiusz Milik ended his scoring drought in European competition after 10 minutes at the Stadio San Paolo. Napoli's leading scorer in Serie A — with 14 goals — beat an offside trap before rounding goalkeeper Alexander Walke to score into an empty net.
Eight minutes later, Napoli was in full control after Fabian Ruiz volleyed in, also his first European competition goal of the season.
Jerome Onguene finished the scoring with a second-half own goal.
Eintracht had been the highest-scoring team in the competition so far, but Inter's defense was too difficult to crack for the German side. It even needed goalkeeper Kevin Trapp to save Marcelo Brozovic's penalty in the first half to salvage the draw.
In Zagreb, forward Bruno Petkovic secured the first-leg win for Dinamo, converting from the spot in the first half.
Zenit's Sardar Azmoun canceled out Villarreal's opening goal in the first half but the Spanish team scored through Manu Morlanes and Gerard Moreno hit in the second half to take firm control ahead of the return leg.
Forward Rodrigo Rodrigo netted twice in the first half for Valencia, but Viktor Claesson pulled one back in the second half for Russia's Krasnodar.
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