London, England (SportsNetwork.com) - World No. 1 superstar Rafael Nadal and reigning champion Novak Djokovic were Tuesday's round-robin winners at the $6 million ATP World Tour Finals -- the prestigious eight-player season-ending event.
The marquee match of Day 2 saw the second-seeded Djokovic outlast sixth- seeded fellow former No. 1 star Roger Federer 6-4, 6-7 (2-7), 6-2 in 2 hours, 22 minutes. Djokovic, who overcame a second-set hiccup en route to victory, popped nine aces and tallied five service breaks, compared to only two breaks for the once-mighty Federer.
Djokovic broke Federer to open the third set and cruised from there.
The 26-year-old Djokovic is now 15-16 lifetime against the 32-year-old Federer, including a victory just three days ago in a semifinal at the Paris Masters.
The two-time Tour Finals champion Djokovic was last year's big winner at The O2, beating the six-time champ Federer in the final.
The red-hot Djokovic beat David Ferrer in last week's final at the Paris Masters and has now won his last 18 matches overall on tour.
Meanwhile, the former Tour Finals runner-up Nadal whipped his third-seeded fellow Spaniard Ferrer 6-3, 6-2 in 74 minutes at London's O2 Arena. Nadal broke Ferrer six times, compared to only two breaks for the loser.
Ferrer upset his fellow countryman Nadal in a semifinal last week in Paris.
Tuesday also marked a rematch of this year's French Open final, which was won by the formidable Nadal, who now leads his lifetime series with Ferrer 21-5.
The 27-year-old Nadal needs only one more win this week to finish as the year- end No. 1 for the third time in his career.
This week's four-player Group A features the U.S. and French Open champion Nadal (1-0), the Roland Garros runner-up Ferrer (0-1), fifth-seeded former Wimbledon finalist Tomas Berdych (0-1), and seventh-seeded Stanislas Wawrinka (1-0).
Group B is comprised of the Australian Open titlist Djokovic (1-0), fourth-seeded former U.S. Open champ Juan Martin del Potro (1-0), the 17-time Grand Slam winner Federer (0-1), and eighth-seeded Richard Gasquet (0-1).
On Wednesday, Nadal will face Wawrinka and Ferrer will encounter Berdych.
Djokovic and Federer are the only former champions in the field. Nadal was the runner-up in 2010; Ferrer was the runner-up in 2007; and del Potro was a runner-up in 2009.
Round-robin play will be staged through Saturday, with the semifinals on Sunday, and the final next Monday. The top-two players from each group will qualify for the semis.
An undefeated champion this week would earn $1.923 million.