Karen Khachanov thrilled the Moscow crowd as he saved five match points before overcoming veteran Philipp Kohlschreiber at the Kremlin Cup.
The world number eight, top seed and defending champion at the ATP 250 event, found himself 5-3 down in the decider but refused to buckle as Kohlschreiber eyed a notable scalp on his 36th birthday.
Khachanov – now the focus of home hopes after Daniil Medvedev's withdrawal due to fatigue - repelled three match points to force a tie-break and his calmness from the baseline saw him through more trouble to prevail 3-6 6-3 7-6 (9-7).
Andreas Seppi is up next for Khachanov after the experienced Italian similarly came from a set down to beat Roberto Carballes Baena 5-7 6-1 6-3.
Serbian fourth seed Dusan Lajovic also went the distance against Lukas Rosol, coming through 6-4 6-7 (8-6) 6-3 to beat his Czech opponent and set up a quarter-final clash against last year's runner-up Adrian Mannarino – a straight-sets winner over Mikhail Kukushkin.
Stan Wawrinka returned to action at the European Open, competing for the first time since his US Open quarter-final loss, and his troublesome knee was given a thorough workout by Feliciano Lopez.
The two seasoned campaigners provided plenty of entertainment - a sumptuous backhand half-volley at the net in the first set serving as a particular highlight from Wawrinka, who came through 6-7 (4-7) 6-4 7-6 (7-4).
The 34-year-old Swiss will take on Gilles Simon in the quarter-finals after the world number 47 came through his all-French clash against Jo-Wilfried Tsonga 6-4 7-5.
Andy Murray will face Pablo Cuevas in round two after the eighth-seeded Uruguayan beat Hugo Dellien 6-4 6-3, while Guido Pella ensured further South American success by beating Kwon Soon-woo 7-5 7-5 to earn a quarter-final place.
Frances Tiafoe's reward for breaking a three-match losing streak, defeating Yannick Maden, will be a meeting with another German opponent in Jan-Lennard Struff.
At the Stockholm Open, US Open semi-finalist and second seed Grigor Dimitrov was dumped out by Sam Querrey, losing 6-7 (7-9) 6-3 7-6 (7-3). Taylor Fritz was another seed to fall – 7-5 6-4 against Yoshihito Nishioka,
Filip Krajinovic is up next for Nishioka after beating Dan Evans 7-5 2-6 6-3.