Wimbledon champion Goran Ivanisevic dented Gustavo
Kuerten's hopes of claiming the year-end top ranking when he defeated him to win their Masters Cup match 6-2, 6-7, 6-4 in Sydney.
Ivanisevic is only 13th in the Champions' Race and made it into the elite eight-man tournament by virtue of his extraordinary Wimbledon victory which he achieved as a wild card entrant.
The other match in the session saw Yevgeny Kafelnikov fight his way to a 4-6, 6-1, 7-6 (7/5) win over rising Spanish star Juan Carlos Ferrero.
Kuerten's loss opens the way for Lleyton Hewitt and Andre Agassi in their bid to end the year as world number one.
The Brazilian leads the Champions' Race by 48 points from Hewitt, with Agassi another 39 points adrift in third.
Hewitt and Agassi meet in a crucial round-robin match on Wednesday.