Federer had been hoping to get back on the winning track beating del Potro.  Ever since del Potro returned to tour after nearly a year off from playing, he has slowly, but surely been ascending up the ranks.  Federer has, in the meantime, been able to beat del Potro on a regular basis, biding his time that either del Potro would never regain where he was in 2009.

Federer knew, pretty much, that with his 2 wins and both in straight sets, that he would be in the semis regardless.  On the other hand, del Potro knew he needed a win to get in, and despite needing three sets to beat Federer, he secured the win.

The reason is a bit complex and owes to the many scenarios and arcane rules in place.

First, the two players with the best records enter.  If the two are tied with the same record, then head-to-head breaks the tie.  If there is a 3-way tie, then, percentage of sets one is used to rank the three players.  The top 2 make it provided there is no tie between the second and third player.  If there is a tie between any two players, the head-to-head breaks the tie.  If there is a 3-way tie in percentage of sets won (which there was in 2009), then percentage of games won is the next tiebreak, again, if 2 players tie (but there is no 3-way tie), head-to-head in the round robin breaks the tie.

Finally, if there is still a 3-way tie in the percentage of games won, the rankings entering into London are used to decide which 2 move on.

The first tiebreak is percentage of sets won.  Federer won 5 sets and lost 2 for a ratio of 5 sets won to 7 played or 71.4%.  del Potro won his match over Tipsarevic in straight sets, lost to Ferrer in three sets, and beat Federer in three sets, so he won 5 sets and lost 3 sets, for a ratio of 5 sets won to 8 sets played or a percentage of 62.5%.

David Ferrer, so far, has won one match in three sets (over del Potro) and lost one match in straight sets to Federer.  Even if Ferrer beats Tipsarevic in straight sets, he would have won 4 sets and lost 3 sets of won 4 sets out of 7 potentially played (so it hurt that he didn’t win a set off Federer or didn’t beat del Potro in straight sets).  That ratio would be 57.1%, which is less than what del Potro currently has, so even a straight set win over Tipsarevic won’t matter.

Given this rule, if Ferrer wins, there will be a 3-way tie and Federer wins the group because of percentage of sets won and del Potro is runner-up.  If Tipsarevic wins, then del Potro wins the group because he and Fed would be in a 2-way tie, and head-to-head breaks that tie.  Thus, there’s still some

Although Ferrer can’t make the semis, where winning does help Ferrer is in two spots.  First, he would pick up 200 points for the win.  For one match to be worth nearly an ATP 250, that should be incentive enough.  Second, he also earns prize money.

Ferrer does have a negative incentive.  Spain, once again, is in the Davis Cup final.  Despite saying he would beg out of Davis Cup this year, perhaps the change in coaches to Alex Corretja convinced Ferrer to stay.  He’s certainly the main guy they’ll be depending on against the Czech Republic.   And the Spaniards, after struggling to make Feliciano and Fernando a viable doubles team, have now found a team that is good enough to play in London: the duo of Granollers and Lopez.  Granollers is also good enough, in a pinch, to play singles.  A viable doubles team means that Spain could close out matches sooner and thus avoid having its top players play in reverse singles.

In the past two years, Federer has won the championships undefeated.  With that comes the full 1500 points.  With a single loss, the best he can do this year is 1300 points since each round robin win counts 200 points, and he picked up 400 points instead of 600, so lost 200 points he could have picked up.  Djokovic, who is undefeated, still has a chance to win the full 1500 points.

Because of this scenario, Ferrer may not be fully motivated to win, and he’s down two breaks with Tipsarevic trying to serve out the set.  Tipsy had been ill earlier in the week, but he seems to be looking better now.

So we don’t know the semis until it is determined whether Ferrer wins or Tipsarevic wins.