Grigor Dimitrov and Marin Cilic became the latest players to qualify for the ATP Finals on Tuesday after rival Sam Querrey lost in the first round of the Vienna Open.
The pair booked their spots in the end-of-season showpiece event at London's O2 Arena after Querrey joined former champion David Ferrer and fellow seed Kevin Anderson in making an early exit from the ATP 500 event at the Wiener Stadthalle, where home favourite Dominic Thiem advanced.
Ferrer won this competition in 2015 and was a runner-up to Andy Murray 12 months before that.
But the Spaniard will not progress beyond the first round this year, Murray's fellow Briton Kyle Edmund inflicting a 6-2 7-6 (7-5) loss.
Veteran Ferrer was not the only casualty in the Austrian capital, where qualifier Guillermo Garcia-Lopez ousted South African seventh seed Anderson 6-4 6-4.
And Garcia-Lopez's fellow Spaniard Albert Ramos-Vinolas saved four match points to defeat sixth-seeded American Querrey 3-6 7-6 (9-7) 7-6 (7-3).
That result means Dimitrov and Cilic have followed Rafael Nadal, Roger Federer, Alexander Zverev and Thiem in securing an ATP Finals berth.
Thiem, seeded second for the Vienna Open, bucked the trend of upsets by dispatching Andrey Rublev 6-4 6-3.
Another Austrian in the shape of qualifier Dennis Novak joined his compatriot in advancing thanks to a 7-6 (7-4) 7-5 win over Italian lucky loser Thomas Fabbiano.
Sebastian Ofner was unable to make it a hat-trick of Austrian players through to the second round, Lucas Pouille inflicting a routine 6-3 6-4 defeat on the wildcard.
Jan-Lennard Struff was a straight-sets winner over Pablo Cuevas, while fourth seed Pablo Carreno Busta saw off Guido Pella 6-2 7-6 (9-7) in the final match of the day.