Former champion Marin Cilic fell at the first hurdle at the Western & Southern Open, while Andy Murray's singles return ended in defeat.
Cilic, who won the tournament in 2016, lost to Radu Albot in a shock at the ATP Masters 1000 event in Cincinnati on Monday.
Three-time grand slam champion Murray played his first singles match since January but went down to Richard Gasquet.
CILIC CAN'T STOP ALBOT
Seeded 14th and with history in Cincinnati, Cilic was a surprise 6-4 7-6 (8-6) casualty in the first round.
Cilic – the 2014 US Open champion – had six set points in the second set but the Croatian star was unable to convert any.
Albot will face either Laslo Djere or Diego Schwartzman in the second round.
MURRAY MAKES COMEBACK
Former world number one Murray returned to the singles circuit but lost 6-4 6-4 to French veteran Gasquet.
Murray had not played a singles match since January's Australian Open after undergoing hip resurfacing surgery, which left the former world number one's stellar career in serious doubt.
The 32-year-old made his long-awaited comeback on Monday, with the British star later announcing he will skip the upcoming US Open at Flushing Meadows.
QUERREY SETS UP DJOKOVIC CLASH, FED AWAITS LONDERO
Wildcard Sam Querrey faces a huge task after prevailing 7-6 (7-2) 7-6 (7-1) against Pierre-Hugues Herbert.
Querrey will meet world number one Novak Djokovic in the second round of the Masters event.
Djokovic, who is preparing for his first singles match since winning Wimbledon, featured in the doubles on Monday – the top seed teaming up with fellow Serbian Janko Tipsarevic in a 6-2 6-3 loss to Lukasz Kubot and Marcelo Melo.
Another wildcard, Juan Ignacio Londero, also has a mammoth challenge ahead following his 7-6 (7-3) 6-3 victory over Matteo Berrettini.
Swiss maestro Roger Federer – the third seed – is next for Londero.
KYRGIOS TEAMING UP WITH TSITSIPAS AFTER SONEGO WIN
Nick Kyrgios earned a clash with Rogers Cup semi-finalist Karen Khachanov after defeating Lorenzo Sonego 7-5 6-4.
Kyrgios, though, turned his attention to Tuesday's doubles. After teaming up with Stefanos Tsitsipas at the Citi Open, the pair will unite again for a showdown against top seeds Juan Sebastian Cabal and Robert Farah.
"I actually didn't get him a present. I didn't know until I saw it on Twitter this morning, but I don't know. I might get him something small tomorrow," Kyrgios said of the Greek birthday boy. "Maybe a shoelace or something."