Andre Villas-Boas's Russian league leaders Zenit St Petersburg face their first serious test this season as they look to defend their perfect record against FC Krasnodar on Saturday.

Reigning champions Zenit have won all four of their Premiership encounters this season to sit top of the table on goal difference ahead of last season's runners-up CSKA Moscow.

However, Villas-Boas is concerned that new rules, preventing sides from fielding more than six foreign players at any one time, are stopping him from fielding a full-strength team.

"You know that (Russia's sports minister Vitaly) Mutko has changed the rules a week before the start of the season, so now our best players are sometimes forced to deal with this," the former Porto, Chelsea and Tottenham Hotspur manager said.

"It's awful, but that's the reality. I think that everyone wants to see the best on the pitch but we have to give this up.

"Sometimes you have to make sacrifices. But unfortunately that's the reality that we have to adjust to."

FC Krasnodar coach Oleg Kononov said his side will be better prepared for the clash than they were a week ago when narrowly avoiding defeat in their city derby against Kuban.

"I was happy when we saved the day in the derby with Kuban as we lacked freshness after an away Europa League qualifier," said Kononov, whose side finished third last season.

"I hope that the full rest will allow us to play at our best against Zenit."

CSKA will open the weekend schedule on Friday with a clash against Spartak in Moscow's main derby.

Spartak have 78 wins to their name, the Red Army side have chalked up 64 wins and 36 of their meetings, dating back to 1922, have ended all square.

CSKA boosted their line-up by re-signing Ivory Coast marksman Seydou Doumbia, twice previously crowned Russia's top scorer, on loan from Roma ahead of the derby.

"Doumbia is the best forward among those that we could get," CSKA head coach Leonid Slutsky said.

"His arrival is a serious boost for our attacking line. But the derby with Spartak will definitely be hard. Anything can happen in this match."

Victory on Friday would not only allow CSKA to maintain their perfect record but would also boost morale ahead of next week's Champions League play-off encounter at Sporting Lisbon.

Spartak manager Dmitry Alenichev said matches against CSKA are always very special.

"We are all set to produce our best form and top spirit for Friday's meeting," Alenichev said.

"Every one of us has a clear understanding of what kind of match it will be. CSKA are currently in top condition and have yet to concede a goal in Premiership matches. But we will give them a battle, that's for sure."

Fixtures (all times GMT)