Arsenal made Leeds pay for their profligacy with a host of wasted first-half chances before Reiss Nelson scored the winner 10 minutes into the second period.
Unai Emery insisted his side's Europa League semi-final against Valencia is still "50-50" despite Arsenal taking a 3-1 first leg lead to Spain.
Alexandre Lacazette's stunning 36th-minute free-kick put Arsenal through to a Europa League final four meeting with Spanish side Valencia.
Arsenal climbed two points above fourth-placed Tottenham and fifth-placed Manchester United to bolster their chance of qualifying for the Champions League.
Both Premier League clubs Arsenal and Chelsea were initially drawn to be at home in the first leg.
A video has emerged that appears to show a number of Arsenal stars inhaling nitrous oxide.
Arsenal turned on the style against Fulham, recording their biggest win of the season.
Antoine Griezmann's away goal eight minutes from time is a body blow to Arsene Wenger's hopes of bowing out on a high in the final on May 16.
Arsenal quickly ended any doubt over the contest on the field as Aaron Ramsey and Alexandre Lacazette both scored twice in the first half to surely put the tie out of CKSA's reach.
With the 2017 Premier League season all set to begin, here's a look at some of the new signings that can make a splash.
Alexandre Lacazette won the award, voted for by his peers, ahead of PSG trio Zlatan Ibrahimovic, Javier Pastore and Marco Verratti after a remarkable season in which he has scored 27 Ligue 1 goals to help Lyon finish second and qualify for next season's Champions League.