Manchester City boss Pep Guardiola expects the race in Champions League Group C for the last-16 qualification spots to go down to the final round of matches.
Guardiola's men won their first game in the pool 4-0 at home against Borussia Monchengladbach, while his former club Barcelona thrashed Celtic 7-0.
Things were then much tighter on Wednesday as matchday two saw City fight back from a goal down three times to draw 3-3 with Celtic at Parkhead, and Barca come from behind to win 2-1 in Monchengladbach.
Second-placed City's next two fixtures in the group are against leaders Barcelona away and then home, followed by a trip to Monchengladbach before they finish up by hosting Celtic.
And Guardiola, whose side had won each of their 10 games under him in all competitions this season prior to Wednesday's without once trailing, said after the 3-3 draw: "When the fourth team in the pot is Celtic, you immediately realise how tough it will be to qualify, to go through.
"(It will be like that) to the last game, when we play Celtic at home. I said to my players in the beginning, in this group it will be decided if you are able to qualify for the next round in the last game.
"When we play against Barcelona, it will be tough.
"Now is the time to analyse what we did, and we are going to see what happens at Barcelona, and then (against them) at home.
"Of course, we are now second, but we have chances still, so we'll see. We are going to recover well, and focus on White Hart Lane (Sunday's Premier League clash with Tottenham). We have time to think about Barcelona."
In a pulsating game that matched the atmosphere at Celtic Park, the hosts went 1-0 up in the third minute through Moussa Dembele, with Fernandinho then swiftly drawing things level.
Celtic soon regained the lead via a Raheem Sterling own goal, before the England man equalised in composed fashion eight minutes later.
The pattern then continued early in the second half as Dembele put his team 3-2 up in the 47th minute, only for Nolito to make it all square again in the 55th.
Guardiola admitted the contest would serve as a valuable lesson to City, and added: "Thinking about the performance, the guys, how they fought - it doesn't matter about the mistakes.
"How they tried to win the game until the end, I'm so satisfied."
City defender Pablo Zabaleta felt the display had been a good demonstration of the character in the Blues camp.
"To come back three times when we were behind shows the character of the team," said Zabaleta, quoted on City's official website.
"We were a bit unlucky not to score at the end. We created a few clear chances and had control of the game in the second half.
"It's true that we conceded in three key moments of the game, especially the third goal at the start of the second half, but to come back was important.
"We always play to win and of course we should have won. It would have been a great step to qualify for the next round but at this level no games are easy.
"For us it was not the result that we were looking for but when you can't win it is important not to lose."