Theo Walcott scored in the fourth minute as round series on Tuesday with a 1-0 win against Udinese at Emirates Stadium.
Arsenal, which finished fourth in the English Premier League last season, will take the one-goal edge to Italy for the second leg Aug. 24. Udinese was fourth in Italy's Serie A last season.
The Gunners are among 20 teams playing in the playoff round, which will decide the final 10 participants in the group stage.
Walcott's opener followed an inch-perfect, right-wing cross from Aaron Ramsey, who broke free to set up the goal. Walcott finished with a one-time effort to the near post, leaving Udinese goalie Samir Handanovic no chance.
Arsenal struggled to build on the goal, and Udinese threatened a few times and nearly leveled on 82 minutes when Giampiero Pinzi curled a shot just around the right post.
Walcott was denied the clinching goal in the 89th when Handanovic made a great reaction save to push his shot around the right post, but the Italian side was unable to snatch an equalizer in the closing minutes.
Lyon 3, Rubin 1
Lyon, France - Bafetimbi Gomis canceled out an early goal from Rubin, and Lyon went on to a 3-1 win over the Russian side on Tuesday at the Stade de Gerland in the first leg of their Champions League playoff-round series.
Vladimir Dyadyun scored in the third minute for Rubin, but Gomis had Lyon back on level terms inside 10 minutes, and an own goal from Solomon Kverkvelia and Jimmy Briand's second-half strike sealed the French side's victory.
The second leg is at Rubin on Aug. 24. The winner advances to group play.
Benfica 2, Twente 2
Enschede, Netherlands - Oscar Cardozo and Manuel Nolito scored in a 14-minute stretch of the first half, and Portugal's Benfica opened its Champions League playoff-round series with a 2-2 draw at the Netherlands' Twente on Tuesday.
Luuk de Jong scored in the sixth minute as Twente got off to a dream start at De Grolsch Veste but Cardozo scored in the 21st and Nolito scored in the 35th. Bryan Ruiz scored 10 minutes from time as Twente escaped with a draw.
The second leg is at Benfica on Aug. 24. The winner advances to group play.
Viktoria Plzen 3, FC Copenhagen 1
Copenhagen, Denmark - Vaclav Pilar and Martin Fillo scored and Viktoria Plzen of the Czech Republic opened its Champions League playoff-round series against Denmark's FC Copenhagen with an impressive 3-1 win Tuesday at Parken.
Solvi Ottesen's own goal opened the scoring in the 52nd, and Pilar doubled the lead seven minutes later. Ottesen made up for his own goal in the 69th, but it was Plzen that struck again in the 79th through Fillo.
The second leg is at Plzen on Aug. 24. The winner advances to group play.
Sturm Graz 1, BATE 1
Borisov, Belarus - Manuel Weber scored in the 12th minute and Austria's Sturm Graz tied Belarus' BATE, 1-1, on Tuesday in the opening leg of their Champions League playoff-round series at Borisov Gradski Stadium.
Marko Simic scored in the 59th for BATE. The second leg is Aug. 24 at Sturm Graz. The winner advances to group play.