Former world number one Gustavo Kuerten's hopes of reaching the Las Vegas Open quarter-finals were dented when he lost 6-4, 6-3 to German Benjamin Becker in his opening round-robin match on Wednesday.
The Brazilian, who came through an elimination match to make the round-robin stage, must now beat Czech Jan Hernych to stand a chance of advancing to the last eight.
All three players can still qualify for the quarter-finals.
Kuerten, playing just his sixth match of the year after missing almost the whole of 2006 as he recovered from hip surgery, had a break point at 4-4 in the first set but sent a forehand return long.
Eighth seed Becker, the man who ended Andre Agassi's career at the U.S. Open last year, then broke in the next game. A break in the eighth game of the second set was enough to give him the match.
The Brazilian, ranked 804, must beat Hernych in straight sets to stand any chance of advancing. If world number 83 Hernych wins the match or even a set, he will go through.
Spaniard Fernando Verdasco became the first man through to the quarter-finals when he beat Danai Udomchoke of Thailand 6-1, 5-7, 6-2.
In the last eight, the fifth seed will meet either third seed Marat Safin or another Russian, Igor Kunitsyn, who won his opening round-robin with a 6-2, 6-2 win over Austrian Stefan Koubek.
Another Austrian, Juergen Melzer, won his opening match with a 6-4, 2-6, 7-5 victory over Frenchman Michael Llodra.
