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 Carreno 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has been hampered by a hip injury and the 31-year-old looked rusty on Monday, after skipping the Rogers Cup having withdrawn from the Citi Open quarter-finals.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.
|Lucas Silva sets sights on second chance at Real Madrid|
|Saints stalwart Wood signs new deal|
|Pique criticises Real Madrid´s attitude towards refereeing|
|Khedira out for a month after heart treatment|
|Halep pulls off Tsurenko repeat to reach last eight|
|Manchester United are more than just ´world class´ Pogba, says Young|
|Barcelona need to recover confidence, admits Ter Stegen|
|Record-breaker Taylor hungry for more runs|
|Federer to make Madrid Open return|
|Cricket coach Ansari given 10-year ban|
|Chelsea will give everything to turn season around - Azpilicueta|
|Lomachenko to face Crolla in Los Angeles|
|Morata accepts fan hostility: Not even Jesus could please everyone!|
|I have everything in Madrid - James hints at return to Spain|
|The Ashes: Assessing England and Australia ahead of the 2019 series|
|Rebels captain Haylett-Petty re-signs until 2021|
|Ronaldo eyes end to European drought - Champions League in Opta numbers|
|City Football Group buys third-tier Chinese club Sichuan Jiuniu|
|New Zealand crush Bangladesh to seal series sweep|
|CONCACAF Champions League Review: Toronto trounced 4-0, Dynamo win|
|Thiem crashes out in Rio, Del Potro makes winning return|
|Liverpool can ´hurt´ Bayern in Munich, says Henderson|
|Manfred credits Athletics despite missing out on Murray|
|Man United must be wary of Liverpool too – Robertson|
|Machado joining Padres would be great - MLB commissioner|
|Martinez: We hope Icardi returns soon|
|Taylor becomes New Zealand´s all-time leading run-scorer in ODIs|
|PSG star Neymar cried for two days after foot injury|
|Klopp: Liverpool´s final pass not good enough|
|Valverde not worried about Suarez drought|
|Kovac not celebrating after holding Liverpool|
|QPR 2 West Brom 3: Livermore seals last-gasp win|
|Riley on LeBron leaving Heat: I saw a dynasty fly out the window|
|Klopp content with Liverpool´s ´good´ result against Bayern|
|Valverde accepts 0-0 a ´dangerous´ result to take back to Camp Nou|
|Vaughan outlines benefits of home Cricket World Cup for England|
|Three-time champion Tsonga out in Marseille|
|Leclerc impresses to make Ferrari fastest again|
|Van Dijk not missed as patched-up Liverpool stifle Lewandowski|
|Liverpool 0 Bayern Munich 0: Forwards fail to fire in Anfield stalemate|
|Lyon 0 Barcelona 0: Messi off colour as Valverde´s men are held|
|White Sox in ´disbelief´ after Machado reportedly picks Padres|
|Burrell to switch codes and join Warrington|
|Atletico boss Simeone not thinking about Ronaldo|
|Svitolina advances but Osaka is stunned in Dubai|
|Joshua and Miller trade insults at fiery MSG news conference|
|Machado agrees 10-year, $300m deal with Padres - reports|
|Allegri aware Ronaldo gives Juventus Champions League advantage|
|Guardiola labels Bernardo Silva Portugal´s biggest star|
|Khedira out of Juve squad to face Atletico due to irregular heartbeat|
|Manchester City quadruple ´nearly impossible´, says De Bruyne|
|Lovren out of Bayern clash but Firmino included|
|Flipkens withdraws as Larsson is beaten in Budapest|
|100 days to go: Five key questions ahead of the Cricket World Cup|
|It is time to move on - Antonio Brown meets with Steelers owner|
|Reported Milan target Saint-Maximin punished by Nice for missing Angers game|
|Out-of-sorts Osaka upset in first match as number one|
|Sarri´s ´different´ use of Kante a good thing, claims France coach Deschamps|
|Itoje absence confirmed for England´s trip to Wales|
|Ballack expects ´masterful win´ for Man City against Schalke|
|Deschamps salutes resurgent Martial|
|Former Cowboys QB Tony Romo to play Dallas PGA Tour event|
|Halfpenny and Biggar back in training for Wales ahead of England clash|
|Tedesco urges patience with ex-City youngster Matondo|
|Mbappe workload a concern for PSG boss Tuchel|
|Henderson joins Ireland squad for Rome trip|
|Wenger was convinced Ramsey would stay at Arsenal|
|Halep too strong for Bouchard in Dubai|
|Italy captain Parisse to miss Ireland match|
|Philander to miss second Sri Lanka Test with hamstring injury|