Madrid - Still savoring its memorable win against Real Madrid, Barcelona returns home to face Roma on Tuesday with a chance to reach the knockout stage of the Champions League.
Barcelona needs only a draw to guarantee first place in Group E with one round yet to be played.
The Spanish club has 10 points from four matches, five points more than second-place Roma. Third-place Bayer Leverkusen, which plays at BATE Borisov in Belarus, has four points, one more than its opponent on Tuesday.
Barcelona is boosted by the emphatic 4-0 win over Madrid at the Santiago Bernabeu, a result that allowed it to open a four-point lead over second-place Atletico Madrid in the Spanish league with Real in crisis a further two points behind.
Roma, which missed a chance to move near the top of the Italian league after a 2-2 draw at Bologna on Saturday, must find a way to beat Barcelona to keep from dropping to third place with one round left in the group stage.
Here are other things to watch for:
Lionel Messi is expected to be in Barcelona's starting lineup for the first time since tearing a left knee ligament in a Spanish league game almost two months ago.
Messi came off the bench against Madrid in the 56th minute, with his team already up 3-0 at the Santiago Bernabeu.
Coach Luis Enrique kept him on the bench after saying the Argentine playmaker was not yet fit to play a full match, but Messi should be set to go from the start on Tuesday at the Camp Nou, when Barcelona will try to extend its eight-match unbeaten streak.
Luis Suarez and Neymar have thrived in Messi's absence, scoring 23 of the team's 27 goals in all competitions.
Barcelona is trying to reach the knockout stage of the Champions League for the 12th straight year.
Spanish authorities will be on heightened alert for the first game at the Camp Nou since the attacks in Paris.
Barcelona announced special security measures for Tuesday's game. Fans must have a ticket or a season pass to enter the stadium grounds, and nobody will be allowed near the venue carrying backpacks, suitcases or large bags.
DEFENDING ITS GROUND
Roma stayed in contention in the Champions League thanks to a late win over Bayer Leverkusen in its previous game, but a loss or a draw on Tuesday could send the Italians back to third place going into the decisive round, when it hosts BATE Borisov.
"We're going to have to take a step up because it's a different game in a different setting," Roma right back Maicon said. "Hopefully we can prepare for it well and produce a big performance at Barcelona."
Roma is trying to advance to the round of 16 for the first time since the 2010-11 season, when it was eliminated by Shakhtar Donetsk.
Bayer Leverkusen and BATE Borisov risk being eliminated from the tournament if they draw their match in Belarus and Roma beats Barcelona in Spain.
A loss will automatically eliminate BATE, but Leverkusen could still have a chance to qualify despite having to face Barcelona in Germany in the final round.
Leverkusen is coming off a 3-1 win at Eintracht Frankfurt in the Bundesliga, a result that halted a three-game losing streak in all competitions.
Javier "Chicharito" Hernandez scored twice in Leverkusen's win. The Mexican striker has found the net in each of his last seven games, scoring a total of 10 goals.
BATE, which won its 10th straight Belarusian title last month, is trying to reach the round of 16 of the Champions League for the first time.