Roger Federer got off to the worst possible start in the ongoing ATP Finals tournament in London as he faced a straight-set defeat to seventh seed Kei Nishikori in their opening match on Sunday.

Federer, the second seed, headed into Tuesday's match against sixth seed Dominic Thiem in a must-win situation.

Much to the respite of his fans, Federer made light work of Thiem — a 6-2, 6-3 win to keep his hopes of qualifying for his 15th semi-final at the year-ending tournament.

Despite the comfortable win over Thiem, Federer hasn't guaranteed a place in the last-four round as the Lleyton Hewitt group of ATP Finals is throwing up several permutations and combinations.

Group standings

Player Played Win-Loss Sets Games
Kevin Anderson 2 2-0 4-0 25-10
Kei Nishikori 2 1-1 2-2 14-21
Roger Federer 2 1-1 2-2 21-18
Dominic Thiem 2 0-2 0-4 14-25

How is the qualification decided in case of two-way or three-way tie

  • The player with the greatest number of match wins qualifies.
  • If two players are tied at the same spot, then the head-to-head results (ongoing tournament) will decide the qualifier.
  • If three players are tied, the highest percentage of sets won will be considered. If that is also the same for the three in consideration, the highest percentage of games won will be taken into account. If the two criteria are unable to point out the qualifier, the players' ATP rank will be considered.

Matches to be played in the Hewitt group (on November 15)

  • Federer vs Anderson
  • Thiem vs Nishikori

Possible qualification scenario for Federer

Roger Federer
File photo of Roger FedererJulian Finney/Getty Images

If Federer beats Anderson in two sets and Thiem beats Nishikori, the Swiss great will qualify along with his South African opponent.

If Federer beats Anderson in two sets and Nishikori takes three to beat Thiem, then Federer and Anderson make the cut.

If Federer wins in three and Thiem beats Nishikori, the 20-time Grand Slam champion still qualifies.

Federer can qualify even if he loses tomorrow. Yeah, you read that right.

Federer will go through along with Anderson if he loses in three sets and Thiem beats Nishikori in straight sets.

On the other hand, Federer may not qualify even if he wins tomorrow's match.

Federer will not qualify if he takes three sets to beat Anderson and Nishikori beats Thiem (regardless of the no of sets taken by the Japanese).

On the other hand, if both Federer and Nishikori clinch straight-set wins tomorrow, then it will be a three-way tie between Anderson, Federer, and Nishikori.