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has been hampered by a hip injury and the 31-year-old looked rusty on Monday, after skipping the Rogers Cup having withdrawn from the Citi Open quarter-finals.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.
|Cech accepts performances key to securing Arsenal stay|
|Monchi rules out Spain return amid Barcelona rumours|
|Durham skittled for 61 and 66 on the same day|
|Kane isn´t a machine – Lamela defends misfiring Spurs star|
|Hazard left out of Chelsea squad for PAOK trip|
|Tiger feels close to ending drought after achieving Tour Championship goal|
|Leno in line for Arsenal chance in Europa League|
|Gazidis leaves Arsenal in good shape, insists Emery|
|Eagles bring back WR Matthews, place Wallace on IR|
|England include Stokes, Hales and uncapped seamer Stone in ODI squad|
|Suwon Bluewings 0 Jeonbuk Motors 3 (aet, 4-2 on penalties): Quarter-final comeback ends in spot-kick|
|Australia captain Paine ready to slog it out with Pakistan|
|Nadal pulls out of China Open & Shanghai Masters with knee injury|
|Ostapenko begins Korea Open defence with victory|
|Joshua v Povetkin: How Olympic super heavyweight champions have fared|
|Liverpool ´made a point´ with PSG victory - Gomez|
|Osaka wows Tokyo crowd on WTA return|
|Excited Povetkin arrives in England for Joshua showdown|
|Kovac patience with Sanches set to reap rewards|
|Kovac: Madrid strong without Ronaldo, Messi is the best|
|ATP suspends umpire Lahyani over Kyrgios ´pep talk´ at US Open|
|North and South Korea seek to co-host 2032 Olympics|
|Ampadu signs long-term Chelsea contract|
|Don´t blame Neymar, PSG players all guilty - Thiago Silva|
|Real Madrid seek to plug Ronaldo scoring gap – Champions League in Opta numbers|
|Brewers allow Cardinals to inch closer with loss to Reds|
|Manchester is still red – Depay ahead of Lyon´s clash with City|
|Orioles become first pro team to wear braille jerseys|
|Klopp hails ´brilliant´ Henderson|
|PSG attack lacked aggression, says Tuchel|
|Atletico Tucuman 0 Gremio 2: Libertadores holders draw first blood in QFs|
|Thomas plays down wrist worry|
|Rakitic wants more from Barcelona hat-trick hero Messi|
|My players aren´t s*** – Pochettino defends Spurs squad|
|Cubs reliever Morrow won´t return this season|
|Tebow expected to be back in Mets organisation next season|
|Inter can compete against anyone – Icardi revels in Champions League victory|
|Tuchel: PSG loss to Liverpool not logical or correct|
|Sturridge in best shape I´ve seen – Klopp|
|Milojevic celebrates ´fantastic´ occasion after Red Star´s 26-year wait|
|Rodrigo and Simeone salute Griezmann display|
|Seahawks DE Clark on Bears QB Trubisky: He´s nothing special|
|Carroll says Wilson has been ´over-trying a little bit´|
|Judge returns to Yankees´ starting lineup|
|Napoli boss Ancelotti takes aim at Red Star time wasting|
|Favre admits young Dortmund stars struggled in Bruges|
|Otavio would trade goal for Porto victory as he bemoans Schalke draw|
|Championship Review: Pacesetters Leeds crush Preston, Villa end five-game winless run|
|Neymar world-class but not a defender - Klopp says Liverpool targeted PSG star|
|Waiters injury update: Heat guard unlikely to be ready for start of season|
|Gojowczyk eliminates returning Tsonga in Metz|
|Liverpool enjoying best start in 57 years after PSG win|
|Galatasaray 3 Lokomotiv Moscow 0: Terim´s side make light work of Russians|
|Club Brugge 0 Borussia Dortmund 1: Super sub Pulisic scores late winner|
|Schalke 1 Porto 1: Fahrmann saves one penalty, but Otavio scores another|
|Red Star Belgrade 0 Napoli 0: Points shared as Italians fail to make possession pay|
|Liverpool 3 Paris Saint-Germain 2: Firmino strikes late as Reds edge five-goal thriller|
|Monaco 1 Atletico Madrid 2: Costa and Gimenez seal comfortable win|
|Chaos reigns in Serie B as league denies suspension claim|
|India secure Asia Cup victory despite valiant Hong Kong effort|
|Messi makes "extraordinary routine", says Valverde after Barca star surpasses Ronaldo|
|Spalletti´s Inter to take heart from Tottenham turnaround|
|Casillas features in landmark 20th Champions League campaign|
|Spurs deserved more from best performance of the season - Pochettino|
|Brown absent from Steelers facility on Monday due to ´personal matter´|
|Toronto´s Bussey given eight-match biting ban|
|Barcelona 4 PSV 0: Marvellous Messi lights up Camp Nou|
|Inter 2 Tottenham 1: Icardi, Vecino complete dramatic late turnaround|
|Allegri, Chiellini defend Costa over spitting ban|
|Firmino only fit enough for bench duty|