QUARTER-FINAL PREVIEW: Two men’s battle to be World No. 1 and two others’ quest to qualify for the Barclays ATP World Tour Finals are among the intriguing storylines at the BNP Paribas Masters quarter-finals on Friday.

Play begins with Top 2 Americans Jack Sock and John Isner squaring off on Court Central. The countrymen captured the ATP Masters 1000 Shanghai doubles title together on October 16 -- one of four finals in four weeks to feature the 24-year-old Sock. Later Friday, Sock will partner Nicholas Monroe in the doubles quarter-finals, marking the Nebraska native’s 33rd match in the past 33 days. Sock is 12-4 in singles and 14-1 in doubles since October 3, with six off-days and now six days during which he has played both singles and doubles.

Following Sock vs. Isner, World No. 1 Novak Djokovic faces familiar foe and fellow Barclays ATP World Tour Finals qualifier Marin Cilic. Djokovic boasts a 14-0 FedEx ATP Head 2 Head record versus Cilic, marking his most victories without a loss against any player on tour. Inside the numbers, the Serbian star has won 72 of 96 games to sweep 12 straight sets against the Croat -- a ratio of three games won by Djokovic for every one game won by Cilic.

Trailing Djokovic in the schedule is the same man trailing him in the race for No. 1. Andy Murray is 415 points behind Djokovic. If Cilic defeats Djokovic for the first time, Murray will rise to No. 1 by beating Tomas Berdych on Friday and either Jo-Wilfried Tsonga or Milos Raonic on Saturday. He would also top the rankings with a win in the final and a Djokovic loss in the semi-finals. Murray, 29, is bidding to become the second-oldest player to debut at No. 1, younger only than a 30-year-old John Newcombe on June 3, 1974. The Brit would own the ATP World Tour record for most time between becoming No. 2 and No. 1, having debuted at No. 2 on August 17, 2009.

Murray, Tsonga, Berdych and Raonic are all former Paris finalists with plenty to play for this weekend. Tsonga will qualify for the Barclays ATP World Tour Finals by winning his second Paris title. Berdych will qualify by advancing to the final, or by reaching the semis as long as Tsonga does not win the title. Raonic, who has already secured his spot in London, will achieve a career-high ranking of No. 3 by reaching his second Paris final in three years.