James Blake was controversially given a place in the Las Vegas Open quarter-finals on Thursday after the ATP reversed a quirk in the rules of its new round-robin format that had seen the American ousted in bizarre fashion.

Needing to beat Argentine Juan Martin del Potro with the loss of no more than five games to advance to the last eight, top seed Blake was leading 6-1, 3-1 when his opponent retired because of breathing problems, handing Blake a walkover.

But Blake was then told he would not advance as the rules stated games won or lost in a walkover did not count. His place would go to Evgeny Korolev on a head-to-head basis after the Russian beat Blake earlier in the competition.

After a lengthy discussion, however, the ATP backtracked and awarded Blake a place in the last eight instead of the Russian.

"James Blake will be awarded the group win on the basis that the rules were not sufficiently explained," ATP executive chairman and president Etienne de Villiers said in a statement.

"James was within just a few games of winning this match comfortably to advance. Juan Martin (del Potro) has stated that he would have completed the match had he been fully aware of the implications of his retirement.

"The ATP will be awarding Evgeny Korolev the amount of $11,375, the average sum of the prize money for the quarter-finals and semi-finals at this event."

Had Blake won a completed match with the loss of no more than five games, all three players would have been level on sets won and the American would have advanced due to a higher percentage of games won.

"It seems to me the right decision," Blake said. "Maybe that's biased but it is a fair decision.

"The ATP are correcting a mistake. It's a solution to a problem, based on common sense.

"We are still in an experimentation process with this round-robin and this is what has happened. There are going to be flaws about it."

Blake will meet Sam Querrey in the last eight after the American teenager beat his compatriot Paul Goldstein 6-4, 1-6, 6-4.

Austrian fourth seed Juergen Melzer crushed Paul Capdeville 6-2, 6-2 to set up a quarter-final with Czech Jan Hernych, who beat former world number one Gustavo Kuerten of Brazil 6-4, 6-4 in his final round-robin match.

Seventh seed Tim Henman, still suffering from the after-effects of a virus, was beaten 6-7, 6-3, 6-4 by Spaniard Feliciano Lopez.

Lopez will play Australian second seed Lleyton Hewitt, who came back to beat Australian Open champion Thomas Johansson 6-7, 6-3, 6-4.

The remaining quarter-final will be between Spaniard Fernando Verdasco and either third seed Marat Safin or fellow Russian Igor Kunitsyn.