After six legs of 13 and heading to Portugal, American Laura Kraut leads the overall standings with 132 points.
With victory worth 40 points, the situation at the top is tight; German Christian Ahlmann is hot on Kraut's heels with 124 points, while Australia's title holder Edwina Tops-Alexander is a single point behind on 123.
Britain's William Funnell (107) and Portuguese Luciana Diniz (106) follow the leading trio.
A host of riders, including Michael Whittaker, Alvaro de Miranda, Meredith Michaels-Beerbaum, Denis Lynch and Marcus Ehning, sit just under the 100-point mark and within striking distance of the leaders, with everything to play for in Estoril.
Dutch duo Gerco Schröder and Harrie Smolders aim to be among the contenders in Saturday's Grand Prix, along with the likes of Swedish riders Rolf-Göran Bengtsson and Henrick von Eckermann.
Spain will be represented by Sergio Alvarez Moya and Marta Ortega Perez, France by Roger-Yves Bost, Kevin Staut and Pénélope Leprévost and Belgium by Judy-Ann Melchior and Ludo Philippaerts.
Home hopes rest with just one rider: Diniz. The Sao Paulo-born naturalised Portuguese has never won a GCT title, but enjoyed her best finish in 2011 at home and will be aiming to go one better this time around.