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has been hampered by a hip injury and the 31-year-old looked rusty on Monday, after skipping the Rogers Cup having withdrawn from the Citi Open quarter-finals.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.
|United States 1 Ecuador 0: USA make it three from three under Berhalter|
|Varane refuses to discuss Madrid future|
|Real Madrid and Barcelona want Skriniar – agency|
|´Focus on the game´ – Bryant to Duke star Zion|
|MLB preview: Projecting AL West|
|Dahmen, Straka share Valspar lead|
|Celtics´ Smart fined $50k after shoving 76ers star Embiid|
|O´Neill wants more goal threat from Northern Ireland|
|Cousins set for Warriors return|
|Koeman applauds Depay but accuses Netherlands of being ´careless´ in 4-0 win|
|This Week in US Sports: MLB handing out tons of extensions, playoffs loom in NBA|
|Hazard: Tielemans will be major player|
|Zion Williamson: Love for Duke led to injury return|
|Andreescu roars back from the brink to pull off stunning Miami victory|
|Warrington cling on to beat Wakefield in thriller|
|Netherlands 4 Belarus 0: Depay inspires winning start to Euro 2020 qualifiers|
|Belgium 3 Russia 1: Hazard spares Courtois´ blushes|
|Northern Ireland 2 Estonia 0: McGinn, Davis get Euro 2020 qualification campaign off to a flyer|
|Paqueta ready for Brazil´s number 10 shirt|
|Robert Griffin III returns to Ravens to back up Lamar Jackson|
|Rays sign Blake Snell to five-year, $50m extension|
|McLeish brushes off Scotland sack talk after dismal defeat|
|Messi to start for Argentina after nine-month absence|
|Khedira cleared to return to training after heart problem|
|Southgate ´won´t hesitate´ to start Sancho, Hudson-Odoi after Rashford injury|
|Timberwolves without Rose, Teague and Covington for rest of season|
|CAS delays decision on Semenya case|
|Rashford out of England double-header|
|Rice apologises for pro-IRA social media comments|
|Kazakhstan 3 Scotland 0: McLeish´s men torn asunder in Astana|
|Bancroft appointed Durham captain|
|Vunipola: Mental weakness not an issue for England|
|China beaten by Thailand in Cannavaro´s first game in charge|
|Ichiro retires: Baseball legend bids farewell in Tokyo|
|LOOK: Inside Tottenham´s new stadium ahead of grand opening|
|Rashford misses full England training|
|Jaime Mata admits Spain call-up is ´a bit of a novelty´|
|Barclay to make long-awaited Edinburgh bow|
|Ronaldo fined but avoids ban after Atletico celebration|
|Archer surprised by rapid rise to England contention|
|Wolff happy to see competitive Red Bull|
|Marchisio feels for Dybala in Juve´s Ronaldo era|
|Pogba relishing partnerships with Mbappe and Rashford|
|Double bogeys spoil Que´s stunning start|
|Pogba: Solskjaer asked us to enjoy football again|
|Burgess double inspires Rabbitohs comeback|
|Crusaders preparing for emotional Super Rugby return|
|FIFA bans referee from football for life for corruption|
|Rockets´ defensive frailties frustrate Harden|
|Odegaard surprised by Zidane´s Real Madrid return|
|Embiid: Smart´s cheap shot inspired 76ers win|
|Target Maguire and don´t be afraid to kick - Vydra reveals Czech Republic´s England plans|
|Henderson vows to win Liverpool, England trophies after near misses|
|Embiid leads 76ers to rallying win over Celtics, Raptors top Thunder in OT|
|Neymar missed by Brazil – Richarlison|
|Angels confirm Trout´s record contract extension|
|Tipsarevic ends ATP drought as rain wreaks havoc|
|Sixers star Joel Embiid: I´m the most unstoppable player in the NBA|
|Azarenka advances before rain hits Miami Open|
|Spurs coach Popovich gives Wade special gifts before final matchup|
|´You just gotta keep pushing´ – Kobe Bryant gives advice to LeBron James|
|Kyler Murray criticism ´garbage´, says Oklahoma team-mate Powers|
|MLB preview: Projecting AL Central|
|Krstajic: Jovic can play for Barcelona|
|Zion Williamson and five other stars to watch during March Madness|
|Low hints Neuer to start Euro 2020 qualifiers as first-choice goalkeeper|
|Low allays Sane injury fears after crude challenge|
|Wales fresh for Slovakia clash – Giggs|
|ES Tunis to face CS Constantine in CAF Champions League quarter-finals|
|Renato Sanches ´not happy´ at Bayern Munich|