Pablo Carreno Busta's bid to reach the ATP World Tour Finals suffered a blow when Daniil Medvedev knocked him out of the Kremlin Cup, but Grigor Dimitrov and David Goffin moved a step closer to qualifying for London on Thursday.
Carreno Busta is vying for one of the four remaining spots in the season-ending showpiece at the O2 Arena next month, but the top seed and defending champion suffered a blow when Medvedev beat him 6-3 6-3 to reach the quarter-finals in his homeland.
The unseeded Medvedev will face qualifier Mirza Basic in the last eight after breaking the Spaniard four times and serving eight aces in an impressive straight-sets victory.
Basic thrashed Laslo Djere 6-1 6-1, while Ricardas Berankis and lucky loser Alexander Bublik, who beat second seed Albert Ramos-Vinolas, also went through in Moscow.
Dimitrov looks to be destined for London after the Stockholm Open top seed ousted qualifier Jerzy Janowicz 7-5 7-6 (7-5) to reach the quarter-finals.
The world number eight from Bulgaria has already claimed three titles this year and could add another as he prepares to face Mischa Zverev, who saw off Viktor Troicki in straight sets.
Kevin Anderson is still in with a chance of playing in the lucrative Tour Finals and saw off Hyeon Chung 6-3 6-2 in the Swedish capital, while Juan Martin del Potro and Yuichi Sugita were also winners.
Goffin has already won two titles this month and a third could be on the cards in his homeland after the top seed defeated wildcard Frances Tiafoe 7-6 (7-1) 6-2 to seal his spot in the European Open quarter-finals.
Wildcard Jo-Wilfried Tsonga, the second seed, got the better of fellow Frenchman Kenny De Schepper 6-4 6-3 in Antwerp.
Diego Schwartzman, David Ferrer and Stefanos Tsitsipas can also prepare for last-eight encounters.