London, England (SportsNetwork.com) - Two-time champion and reigning titlist Novak Djokovic and six-time champ Roger Federer were Thursday's round-robin winners at the season-ending $6 million ATP World Tour Finals.
The second-seeded former world No. 1 Djokovic was tested on Day 4, as he got past fourth-seeded Juan Martin del Potro 6-3, 3-6, 6-3 in 1 hour, 54 minutes. Djokovic smacked 10 aces and broke del Potro on two occasions, while the menacing Argentine settled for only one break in the setback.
Djokovic is now 11-3 lifetime against the 6-foot-6 del Potro, including 4-1 this year. The two played in the longest-ever Wimbledon semifinal this past summer, with Djokovic prevailing in five sets in 4 hours, 43 minutes.
The hot Djokovic improved to 2-0 in group play this week and will appear in Sunday's semifinals in London. The Serbian star beat Federer in last year's Tour Finals finale.
Djokovic has now won his last 19 matches overall on tour, including a title last week at the Paris Masters.
Meanwhile, the sixth-seeded Swiss great Federer cruised past eighth-seeded Richard Gasquet 6-4, 6-3 in 81 minutes at The O2. Federer popped six aces, compared to zero for Gasquet, and the former No. 1 star broke his opponent four times, compared to only one break for his French counterpart.
Federer is now 11-2 lifetime against Gasquet.
The 32-year-old Federer (1-1), who piled up 30 unforced errors on Thursday, still has a chance to reach the semifinals this week. Saturday's del Potro- Federer winner will advance to the final four at the prestigious eight-player event.
This week's four-player "Group A" features U.S. and French Open champion Rafael Nadal (2-0), Roland Garros runner-up David Ferrer (0-2), former Wimbledon finalist Tomas Berdych (1-1), and Stan Wawrinka (1-1).
Friday's Group A round-robin bouts will pit Nadal against Berdych and Ferrer versus Wawrinka. Nadal beat Berdych in the 2010 Wimbledon finale.
"Group B" is comprised of the Australian Open titlist Djokovic, del Potro (1-1), the 17-time Grand Slam winner Federer (1-1), and Gasquet (0-2).
Djokovic and Federer are the only former Tour Finals champions in this week's field. Nadal (2010), Ferrer (2007) and del Potro (2009) are former runners-up.
Round-robin play will be staged through Saturday, with the semifinals on Sunday, and the final on Monday. The top-two players from each group will qualify for the semis.
An undefeated champion this week would earn $1.923 million.