MADRID (AP) — Atletico Madrid has already won Group G, leaving Zenit St. Petersburg to tussle with FC Porto for seocnd.

Porto hosts winless Austria Vienna on Tuesday looking to boost its chances of reaching the last 16. Zenit plays Atletico.

Atletico leads the group with 12 points from four games, while Zenit is second with five points, Porto has four and Austria Vienna has one.

Here are five things to know about Group G:

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TIME DIFFERENCE

Zenit and Atletico's early kickoff time means Porto will know exactly where it stands in the group heading into its 8:45 p.m. (1845 GMT) match against the Austrian champions.

A Zenit victory means Porto must win or it will likely be playing in the Europa League in February. If Zenit fails to win, a Porto victory will see it move into second ahead of its final match at Atletico. That would be a big boost with Zenit next at Austria Vienna.

Porto has lost both of its Group G home games — the two-time European champion's worst run in the competition. Porto has four points and only one win so far.

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REST TIME

Atletico is the only team to clinch its group in the Champions League, and its impressive season continued Saturday with a 7-0 win over Getafe.

With participation in the next round assured, Atletico coach Diego Simeone has left strikers Diego Costa and David Villa in Madrid. That will give backups Adrian Lopez and Leo Baptistao playing time in Russia.

Simeone also left defenders Filipe Luis and Diego Godin, and midfielders Tiago and Arda Turan at home in the Spanish capital.

"We've brought a strong group and we're hopeful the game turns out as we expect it to," Simeone said on Monday. "Football is about competing, earning minutes, winning your place. Words are meaningless, football players have to be ready to play."

Striker Cristian Rodriguez will also be in the squad after arriving separately following the birth of his daughter.

Atletico has outscored opponents 12-2 in the competition.

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FEELING THE HEAT

Austria Vienna's visit to Porto offers the home team a chance to ease the pressure from its disgruntled fans.

Porto coach Paulo Fonseca had to endure jeers, whistles and white handkerchiefs at the end of his team's 1-1 home draw against Nacional last weekend. It was the first time in eight months that Porto dropped points at its Stadium of the Dragon, cutting its Portuguese league lead to one point.

The Portuguese champions aim to recover some confidence against Austria Vienna, the group's weakest member with only one point so far. Porto won 1-0 in Austria in its opening group game.

"Setbacks help us improve," Porto striker Jackson Martinez said.

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PUZZLE SOLVING

Austria Vienna coach Nenad Bjelica is hopeful to have solved the puzzle of his toothless attack.

Instead of fielding either Philipp Hosiner or Roman Kienast, he might now play both strikers after the formation proved successful in the domestic league.

Austria is on a three-match winning streak, scoring nine goals and conceding none. For months, Hosiner has been seeking the kind of form that saw him score 29 goals last season and he has been replaced with Kienast regularly. However, Hosiner played alongside his teammate and found the net three times in a 5-0 win at Innsbruck on Saturday.

"I hope we can take something home from Porto as well now," Hosiner said.

Austria Vienna must beat Porto to stand a reasonable chance of finishing third in Group G.

"We want to show a good match and if we do, we might be able to stage a surprise," Bjelica said.

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RISE UP

Zenit will need to be inspired when it hosts Atletico.

"Our team depends much on enthusiasm," Zenit coach Luciano Spalletti said. "When the team is enthusiastic, it spreads its wings."

Spalletti's side is unbeaten in eight matches at home in Europe but has stumbled in the domestic league, losing its last two matches. The result left the team even with Lokomotiv Moscow at the top of the Russian league standings.

"We will speak to the players and try to improve the situation," Spalletti said after a 2-0 loss to Rostov on Friday. "Plus we need some luck. And we need to be fired up, it's like a gas that helps to move ahead."

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Follow Paul Logothetis on Twitter @PaulLogoAP