No.1 seed Isner battled past American compatriot Tennys Sandgren to secure a win that moved him above Kei Nishikori into ninth in the ATP Finals rankings.

But his ambitions of gaining further ground on No.8-ranked Dominic Thiem were rocked by a 1-6 6-3 6-3 loss to Gulbis.

Former top-10 player Gulbis consequently reached a first ATP World Tour final since 2014, with Next Gen star Stefanos Tsitsipas standing in his way of glory.

No.3 seed Tsitsipas defeated No.2 seed Fabio Fognini, who himself remains an outside shot for the ATP Finals, in a one-sided 6-3 6-2 victory.

At the Kremlin Cup, Adrian Mannarino defeated Andreas Seppi 7-5 7-5 to set up a showpiece with Karen Khachanov, who defeated Daniil Medvedev 6-1 6-7 (5-7) 6-3 in an all-Russian semi-final.

Diego Schwartzman sustained more European Open heartbreak. The Argentinian, who is twice a runner-up in Belgium, lost out 6-3 6-4 to Gael Monfils.

That win for Monfils booked a final against top seed Kyle Edmund, who defeated 2016 champion Richard Gasquet 6-3 6-4.

It marks the second time Edmund has reached at ATP final, the first ending in defeat to Pablo Andujar in Marrakech in April.