?Rafael Nadal of Spain celebrates after defeating compatriot Tommy Robredo in the Paris Masters Series tennis tournament November 12, 2009.[Xinhua/Reuters Photo]

World number two Rafael Nadal looks to unseat Roger Federer as world number one by the end of the season as the gritty spaniard remained in hunt for Paris Masters crown with a battling victory in the round of 16 on Thursday.

Nadal had needed to save five match points to get past Spanish compatriot Nicolas Almagro in a second round marathon of over three hours on Wednesday, and on Thursday he wobbled again against Tommy Robredo, another countryman and a player he had beaten in straight sets in all five of their previous encounters.

"I was a little less lucky than yesterday. I wasn't playing my best today, but I played better than yesterday," Nadal said of the6-3, 3-6, 7-5 win over Robredo.

With Federer losing to Frenchman Julien Benneteau in a shock upset on Wednesday, a win for Nadal in Sunday's final would leave him just 305 points adrift of his arch rival with the season-ending ATP World Tour Finals to come in London later this month.

?Tommy Robredo of Spain returns the ball to compatriot Rafael Nadal during their match in the Paris Masters tennis tournament November 12, 2009.[Xinhua/Reuters Photo]

Nadal ended Federer's long reign as world No.1 after winning the Beijing Olympics last year, but Federer bounced back to regain the top spot in July after winning both the French Open and Wimbledon titles.

Elsewhere, fourth seed Andy Murray lost 1-6, 6-3, 6-4 to Radek Stepanek of the Czech Republic while Russian Nikolay Davydenko became the seventh player to qualify for the eight-man London Tour Finals despite losing 6-3, 3-6, 6-4 to Sweden's Robin Soderling in another third round match.

But Spain's Fernando Verdasco, who was holding on to the eighth and final slot, lost 3-6, 6-3, 6-4 to Marin Cilic of Croatia.

His defeat meant that Davydenko was assured of finishing the week no lower than eighth which stamped his ticket for London.