Novak Djokovic was forced to dig deep as he reached the Cincinnati Masters second round, while Andy Murray was a shock casualty.
Former world number one Djokovic needed nine match points to see off Steve Johnson at the ATP Masters 1000 tournament on Monday.
Earlier in the day, three-time grand slam champion Murray was upstaged by French 16th seed Lucas Pouille in three sets.
Stan Wawrinka outlasted 12th seed Diego Schwartzman, ninth seed John Isner went down to Sam Querrey, Kei Nishikori was too good for Andrey Rublev, while Richard Gasquet was ousted by 13th seed Pablo Carreño Busta.
DJOKOVIC SURVIVES FIRST TEST
The 13-time major winner has won every Masters tournament except for the Cincinnati event.
But 10th seed Djokovic got his bid off to a winning start in a hard-fought 6-4 7-6 (7-4) victory over American opponent Johnson.
Johnson saved eight match points before five-time runner-up Djokovic – who was upset by Stefanos Tsitsipas at last week's Rogers Cup – finally closed out the contest.
Next up for 30-time Masters 1000 titlist Djokovic is Adrian Mannarino.
A two-time Cincinnati champion, former world number one Murray suffered a 6-1 1-6 6-4 loss.
Murray's career has been hampered by a hip injury and the 31-year-old looked rusty on Monday, after skipping the Rogers Cup having withdrawn from the Citi Open quarter-finals.
After Pouille dominated the first set, Murray stormed through the second to level the match, but that was as good as it got for the Scottish star – who was playing just his fourth tournament in 13 months following hip surgery in January.
WAWRINKA TO FACE NISHIKORI
After two knee surgeries, three-time major champion Wawrinka looks to be returning to his best form following Monday's 6-2 4-6 6-3 win.
Ranked 151 after dropping to as low as 263 in June, 2012 Cincinnati semi-finalist Wawrinka has now won three of four matches having tallied a 3-10 record previously.
"Little by little it's coming back... I expect some ups and downs the next few weeks, months, because I still know that not everything is back," the 33-year-old said. "But the most important [thing] is to keep working, keep doing the right thing, keep pushing in matches, keep winning matches like today. Little by little I'm gaining my confidence. I'm finding a way how to play my best tennis. I'm going to put my fitness back. I still have a lot of work to do, but I'm really positive with where I am right now."
Nishikori awaits in the next round after the Japanese star beat Rublev 7-5 6-3.