It's all to play for as Chelsea and Arsenal slugged out a 1-1 draw at Stamford Bridge.
Eidur Gudjohnsen gave the West Londoners a sniff of their first victory in 17 encounters against Arsenal on 52 minutes.
Running on to a long left-footer by Frank Lampard on the left, Gudjohnsen outpaced Sol Campbell in the race for the ball.
Arsenal goalkeeper Jens Lehmann failed to clear the ball and Gudjohnsen capitalised, taking the ball from just outside the Arsenal area on the right, and curled a low, left-footer into the back of the empty net.
"It was a goal for Eidur's determination," said Chelsea coach Claudio Ranieri.
"We then had an excellent ten minutes and were unlucky not to get the second goal." That was true as Lehmannn made up for his error by making two saves, seconds later - first a blaster from Lampard and then the follow-up by Adrian Mutu.
But as Ranieri admitted afterwards: "Arsenal are Arsenal, they are playing like a team on another planet at the moment." And so it proved to be as the North Londoners equalised on 59 minutes. They forged excellent possession, leaving Ashley Cole free on the left to ping in an excellent cross which was met by Robert Pires, who arrowed a header low, past Marco Ambrosio's despairing dive.
"We gave away a goal at an unexpected time," revealed Arsenal manager Arsne Wenger.
"But we showed great determination, patience and character to get level." The match ebbed and flowed as far as possession was concerned, but neither side could manage the knockout punch.
Chelsea skipper Marcel Desailly was red-carded for two yellow offences the first on 81 minutes for knocking the ball out of Lehmann's hand, the second, two minutes later for a foul on Patrick Vieira.
It's still unclear who is favourite to go through to their first Champions League semi-final on Tuesday week at Highbury.
Wenger said: "We will try and score. We know we can win at home and we really want to." A 0-0 draw will be good enough for the Premiership champions elect, but Chelsea have an impressive away record in this competition.
"They will have to kill us to go through," said a defiant Ranieri.
"Maybe they will score, but we are strong too," he added.
Under-fire Ranieri still had time and magnanimity to heap praise on the Chelsea faithful who chanted his name throughout the match.
"I want them to support the team. Managers come and go but Chelsea will remain. That is the important thing," he said.