Sebastian Vettel insisted he cannot take victory for granted in Sunday's Indian Grand Prix after claiming pole position in another Red Bull front row lockout on Saturday.

The 25-year-old German who is bidding to become the youngest treble champion in F1 history said he felt happy to be at the front of the field, but knew plenty of hard work lay ahead.

Victoria Azarenka at last made sure of the year-end world number one ranking, just two matches from the finish of the 2012 season, when she overcame Li Na to reach the WTA Championships semi-finals Friday.

The Australian Open champion from Belarus claimed a 7-6 (7/4), 6-3 win over the Chinese star to set-up a last-four clash on Saturday against Maria Sharapova, the French Open champion.

After the highs of their impressive midweek Champions League triumphs, Borussia Dortmund and Schalke return to domestic action trying to close the gap behind Bundesliga leaders Bayern Munich.

Bayern hold a five-point lead at the top of the German league and host Bayer Leverkusen at Munich's Allianz Arena on Sunday, while Schalke lag seven points behind the leaders, and fourth-placed Dortmund are 12 points adrift.

The leadership of La Liga is in play over two consecutive nights in the Spanish capital this weekend, but without the involvement of Real Madrid.

The defending champions are at Mallorca on Sunday and currently sit in fourth position.

Ryan Giggs lit the fuse on Manchester United's potentially explosive clash with Chelsea by claiming the Premier League leaders aren't the finished article yet.

While Chelsea have made a flying start to their title challenge with seven wins from eight matches to open up a four-point lead over second-placed United, Giggs believes it is too early to crown the west London club as champions elect.

A second half goal from Stewart Downing ensured Liverpool avoided an unwanted record Thursday as the English Premier League outfit edged Russia's Anzhi Makhachkala 1-0 at Anfield to bolster their Europa League hopes.

Downing's well taken 53rd-minute strike means Liverpool avoided two straight European home losses for the first time in their history after their recent hiccup against Italy's Udinese and they now lead Group A with six points from three matches.

Juan Martin del Potro secured his place in the ATP World Tour Finals on Thursday after Rafael Nadal's injury-enforced withdrawal from the eight-man season-ender opened up another vacancy.

The Argentine, who had already increased his qualification hopes for London by winning the Vienna title last weekend, was delighted to make it into the finals for the third time.

World number one Victoria Azarenka remains in suspense as to whether she will capture the year-end top spot after losing to Serena Williams for the fifth time in five meetings this year on Thursday.

The Belarusian's 6-4, 6-4 loss to the American in the WTA Championships means she may have to beat Li Na, the former French Open champion from China, on Friday to be sure of becoming number one at the end of 2012.

David Stern said Thursday that he will retire as commissioner of the NBA on February 1, 2014, exactly 30 years after taking the job and guiding the league and its superstar players to worldwide fame.

Stern, who announced his decision at an NBA Board of Governors meeting, will be replaced by current NBA deputy commissioner Adam Silver.

U.S. ski star Lindsey Vonn dismissed talk Thursday that her campaign to race against men was a mere public relations stunt, although she admitted she only had a slim chance of seeing her wish come true.

"It's a serious wish on my part. I already discussed this a few years ago with my coaches and friends," Vonn told journalists in the Austrian resort of Soelden, where she was preparing to race the giant slalom in the opening World Cup event Saturday.