Andy Murray made a winning start to his ATP grass-court season after a straight-sets victory over Christopher O'Connell at the Stuttgart Open.

The three-time Grand Slam champion, who reached the Surbiton Open semi-finals last week, recovered from a slow start before wrapping up a commanding 6-4 6-3 victory in Stuttgart.

Having missed the French Open to focus his efforts on Wimbledon, Murray lost the opening three games of his clash with the Australian qualifier.

However, the former world number one responded in style, breaking his opponent four times on the way to setting up a last-16 clash with Alexander Bublik.

Bublik, the seventh seed, hit 19 aces and won 36 out of 42 points on his first serve as he overcame Denis Kudla 7-6 (7-3) 7-5.

Nick Kyrgios recorded an impressive 18 aces in his 7-6 (7-3) 6-3 win over Jiri Lehecka, but eighth seed Ugo Humbert was ousted 4-6 7-6 (7-4) 6-4 by compatriot Arthur Rinderknech.

Meanwhile, Botic van de Zandschulp and Tommy Paul were among the seeds to fall at the Rosmalen Grass Court Championships.

Sixth seed Van de Zandschulp went down 4-6 6-2 6-1 against Finland’s Emil Ruusuvuori, and seventh seed Paul was beaten 6-7 (7-4) 7-6 (7-5) 7-6 (7-2) by American compatriot Brandon Nakashima.

But there were no such problems for Karen Khachanov. The world number 23 won 28 out of 30 points on his first serve as he downed Alejandro Tabilo 6-3 6-3.

Reilly Opelka and John Isner will meet in the US Men's Clay Court Championship final in Houston, after they claimed semi-final wins on Saturday.

Opelka kept up his strong early-season form with an imposing 6-3 7-5 victory over Nick Kyrgios in the first semi-final, getting the job done in just one hour and nine minutes.

Opelka slung down 18 aces to 11 from Kyrgios and faced only one break point across the contest.

The 24-year-old Opelka won a title in Dallas in February and followed up with a run to the Delray Beach final, with a third trophy match of the season now awaiting him.

The American world number 18 said he admired the "intangibles" from Kyrgios that "you can't teach."

"He's got one of the best serves in the world and when he turns it on he can come up with some shots that not many guys outside the top 10 can," Opelka said in his on-court interview.

Opelka enjoyed the high-tempo match with Kyrgios, who typically wastes no time between points.

"I prefer to play at that speed rather than, for example, Rafa [Nadal] where it's 50-55 seconds between points," Opelka said.

"It's a totally different speed here and I thought I did a good job managing the whole tempo of the match. I put a lot of returns into play and I came up big with my second serve."

He will face 2013 Houston champion Isner, who came from a set down to defeat defending champion Cristian Garin 4-6 6-3 6-4.

The 36-year-old's serve improved as the match went on and ultimately propelled him to victory, facing four break points in the opening set, one in the second and none in the decider.

Isner also served 17 aces on his way to the final, and won 80 per cent of points on his first serve.

"It's never easy when you have to come back from one set down," he said. "My serve certainly helped me out a lot. I didn't have to hit too many balls there in the end, and I didn't want to.

"I'm so happy to be in the final again here. At my age you never really know when you can get back to a final of a tournament like this."

World number 18 Reilly Opelka is through to the semi-final of the US Men's Clay Court Championships in Houston after defeating qualifier Gijs Brouwer 6-3 7-5.

The giant American, who measures in at six-foot-11, was terrific in the opening set, winning 84 per cent of his successful first serves (16-19) and allowing no break point opportunities.

The second set was much more competitive, as Brouwer broke first to go up 3-1, but Opelka answered instantly and then grabbed the match-winning break when 6-5 up to avoid a tie-breaker.

Opelka will face a fresh Nick Kyrgios in the semi-final after the Australian was gifted a walkover win against Michael Mmoh due to injury.

In a clash between two top-five seeds, world number 29 Christian Garin got the better of world number 13 Taylor Fritz 6-2 6-7 (5-7) 6-3.

What made Garin successful in the first and third sets was his return game, winning more than half of his return points in those sets combined (26-51).

Fritz had chances to get back into the game, but Garin saved 11 of 14 break point opportunities, including six of seven in the deciding third set.

Garin will play American John Isner in the semi-final after Isner triumphed in a three-set battle against Frances Tiafoe 6-4 2-6 6-3.

Isner's trademark serve was inconsistent, and abandoned him in a grim second-set performance, winning just 11 of 21 service points on his way to getting broken twice to allow Tiafoe to force the match into a third frame.

In Isner's two successful sets, he won more than 80 per cent of his first serves (35-41), and allowed no break point opportunities in the third.

Nick Kyrgios continued his strong return to clay courts on Wednesday, defeating Tommy Paul 6-4 6-2 to progress to the quarter-finals of the US Men's Clay Court Championships in Houston.

Before Tuesday's victory over Mackenzie McDonald, Kyrgios had not played on clay since his infamous meltdown against Casper Ruud at the Rome Masters in 2019, where he threw a chair across the court and walked off during a change of ends.

The 26-year-old Australian wild card's serve kept him in relative control, saving all six break points he faced in the 70-minute match, as well as serving 10 aces and winning 88 per cent of points on his first serve.

"I had to be locked in today and I knew that I had to serve well," Kyrgios said post-match. "It's probably one of the better matches I've played on clay in my career, so I'm pretty happy with the way I came out here today, just put my head down and went to work.

"I've been serving really well, so I just wanted to keep to good habits."

The world number 94 will now face Michael Mmoh, who defeated Sam Querrey 6-2 6-4.

Meanwhile, third seed Reilly Opelka did it relatively tougher in the other all-American clash on Wednesday, coming back from a set down to defeat Mitchell Kreuger 3-6 6-2 6-4.

Opelka came into the match having lost just 13 service games in 19 matches this season, but was broken twice in the opening set against Krueger. The world number 18 has been dealing with a right shoulder injury that forced him to retire at the Miami Open against Francisco Cerundolo.

The 24-year-old still managed 15 aces for the match, including a big serve out wide in the Ad court on match point.

"He played well, I think he came out swinging," Opelka said afterwards. "I think he was free, I wasn't. It's a different situation.

"It was a good opportunity for him to play free and get a win and I'm a guy who hasn't always been the most consistent, especially changing surfaces, so I think it was a good win."

He will face Gijs Brouwer, who progressed past J.J. Wolf with a 6-1 6-1 win.

Nick Kyrgios made his return to clay with a win on Tuesday, defeating Mackenzie McDonald 4-6 6-3 6-4 at the US Men's Clay Court Championships in Houston.

It was the first time Kyrgios had played on clay since his infamous meltdown at the Rome Masters in 2019, where he threw a chair across the court and walked off, handing Casper Ruud a win by default.

After a meandering first set, the Australian world number 94 sparked into life when he saved the first of two break points with a second serve ace in the third game of the second. From there, Kyrgios opened up his shoulders and became more assertive from the baseline, as well as serving 18 aces in the closing two sets.

While again admitting clay is not his favourite surface, the 26-year-old was nevertheless pleased by the manner of the win against McDonald.

"He [McDonald] beat me in Washington where I used to play pretty well, and he's a tricky one because he doesn't make too many errors, and clay's obviously not my preferred surface," he said post-match.

"I just served really well, and just started playing more aggressive and found another gear. I would slip and slide a little bit, but my game actually suits the clay quite well."

While US seventh seed Tommy Paul secured a walkover 6-7 (4-6) 7-6 (7-5) 3-2 win against Peter Gojowczyk, Chilean fifth seed and defending champion Cristian Garin also came from a set down to beat Jack Sock 4-6 6-3 7-5.

American sixth seed Frances Tiafoe faced relatively simpler work, defeating unseeded countryman Marcos Giron in straight sets 6-4 6-4.

Two all-American matches also opened play in Houston on Tuesday, with JJ Wolf securing a 6-4 6-4 win over eighth seed Jenson Brooksby in the first, and Steve Johnson defeating Denis Kudla 6-3 4-6 6-4.

In light of recent high-profile misconduct, the ATP has warned players of harsher punishments for on-court behaviour in an internal note, as it also reviews its framework for stopping repeat offenders.

Nick Kyrgios was fined for audible obscenities and hurling his racquet onto the ground after his defeat to Rafael Nadal in Indian Wells – almost hitting a ballkid - while Alexander Zverev was thrown out of the Mexican Open in February after smashing his racquet repeatedly against the umpire's chair.

Jenson Brooksby also came under fire at the Miami Masters, throwing his racquet and hitting a ballkid in his win over Federico Coria, with Coria also motioning during the match that the consequent point penalty was not sufficient.

The recent spate of on-court outbursts has prompted the ATP to act.

"Effective immediately and as we head into the clay court swing, the ATP officiating team has been directed to take a stricter stance in judging violations of the Code of Conduct," ATP Tour chief Andrea Gaudenzi wrote in a circular to the players.

"Additionally, we are also undertaking a review of the Code, as well as the disciplinary processes, to ensure that it provides appropriate and up-to-date penalties for serious violations and repeat offenders."

Kyrgios was also fined after his loss to Jannik Sinner in Miami for criticising the umpire and smashing his racquet, and although Zverev was handed an eight-week suspension for his outburst in Acapulco, the ATP appears determined to nullify future misconduct.

While Gaudenzi, the Italian former top 20 player, understood the human elements to players competing under extreme pressure, he asserted the game itself should be taken into consideration.

"The first three months of the season have seen an unusual frequency of high-profile incidents involving unsportsmanlike conduct," Gaudenzi wrote.

"These incidents shine a bad light on our sport. This conduct affects everyone, and sends the wrong message to our fans, especially young fans."

Emerging Spanish star Carlos Alcaraz continued his rise with a 7-5 6-3 upset of third seed Stefanos Tsitsipas to reach the Miami Masters quarter-finals on Tuesday.

The 18-year-old, who reached last year's US Open final eight after beating Tsitsipas in the third round, proved too good again for the Greek in one hour and 50 minutes.

Alcaraz reached last week's Indian Wells semi-finals and is now 15-2 on the season, with the win setting up a clash with Serbian Miomir Kecmanovic who upset last week's Indian Wells champion Taylor Fritz 3-6 6-1 6-4.

Tsitsipas broke the Spaniard in the sixth game and led 5-2 before Alcaraz reeled off seven straight games to sensationally take the first set.

Alcaraz surged ahead in the second set, with his defence and speed leaving Tsitsipas short on answers.

The Spaniard, who saved seven of eight break points throughout the match, converted his fourth match point for victory.

"It was really, really tough. He was playing unbelievable," Alcaraz said after the match. "All I can say is I fought until the last ball in the first set [to] come back."

Ninth seed Jannik Sinner won 7-6 (7-3) 6-3 over enigmatic Australian Nick Kyrgios, who had a furious outburst directed at umpire Carlos Bernardes.

The Australian was heard to label Bernardes "an absolute clown" and continued to berate the umpire late in the first set, leading to a code violation and two penalties.

Kyrgios completely lost his cool, demanding to speak to a tournament official before smashing his racquet early in the second set, leading to a game penalty as well.

Sinner will next take on Argentine Francisco Cerundolo after he defeated 28th seed Francis Tiafoe lost 6-7 (2-7) 7-6 (7-3) 6-2.

Top seed Daniil Medvedev made light work of Jenson Brooksby 7-5 6-1 to set up a quarter-final showdown with reigning champion Hubert Hurkacz who won 7-6 (7-3) 6-2 over Lloyd Harris. Medvedev will return to the top of the ATP rankings if he wins their quarter-final.

Second seed Alexander Zverev got past Thanasi Kokkinakis 6-4 6-4 and will meet sixth seed Casper Ruud in the last eight, after the Norwegian won 6-3 6-4 over Briton Cameron Norrie.

Nick Kyrgios called on the ATP Tour to "get new people" after a furious outburst at the umpire in his defeat to Jannik Sinner at the Miami Open.

Kyrgios berated umpire Carlos Bernardes after his walkie-talkie went off mid-point during the first set.

The Australian was heard to label Bernardes "an absolute clown".

He subsequently added: "This is one of the biggest tournaments and you guys can't do your job. It's embarrassing.”

"You wonder why no one watches this sport. It's a laughing stock."

After receiving a code violation for an audible obscenity, Kyrgios was then handed a point penalty for unsportsmanlike conduct in the first set tie-break.

Kyrgios had been appearing to talk with a friend in attendance courtside, with that penalty further infuriating him.

"What's unsportsmanlike? What is unsportsmanlike?" Kyrgios asked of Bernardes.

He then demanded to see a tournament official before smashing his racquet during the opening game of the second set, an action that saw him cop a game penalty and drop his serve.

Sinner maintained the initiative from there, easing to a 7-6 (7-3) 6-3 win, with Kyrgios' mind clearly still focused on the performance of the official even after the dust had settled.

He posted on Twitter: "'You could do the job of the umpire' - what was said to my best friend that got me 'unsportsmanlike code violation' at 5-3 in the first set tie break.

"With 100 of thousands of dollars on the line that was the decision he made because the umpire's feelings got hurt. GET NEW PEOPLE"

Jannik Sinner saved multiple match points for the second straight match on Sunday, advancing to the last-16 at the Miami Masters.

After saving three in his opening match against Emil Ruusuvuoiri, the 20-year-old Italian denied Pablo Carreno Busta five times to eventually win 5-7 7-5 7-5.

A Miami finalist in 2021, the Italian was 5-4 down in the second set before winning 11 consecutive points to force a third frame.

Down a break before breaking-to-love in the third, Sinner was fearless with his ground strokes with the game on the line, moving to 5-0 in deciding sets for 2022.

He will face Nick Kyrgios, who continued his strong start to the year with a 6-2 6-4 win over Fabio Fognini.

Taking only 61 minutes to advance after defeating world number seven Andrey Rublev in straight sets in his opening match, Kyrgios hit 24 winners and only lost eight points on serve.

The Australian wildcard and Sinner were supposed to meet at Indian Wells earlier this year, but the latter had to withdraw due to illness.

Fellow unseeded Australian Thanasi Kokkinakis advanced past Denis Kudla, while Francisco Cerundolo upset Gael Monfils.

In the late result on Sunday, Alexander Zverev was a comfortable 6-2 6-2 winner over Mackenzie McDonald.

The German second seed put McDonald under immediate pressure with his return of service, winning 13 of 22 points on the American's second serve.

Meanwhile, Cameron Norrie and Casper Ruud saw their way through in straight sets.

Nick Kyrgios has defended Emma Raducanu amid a wave of recent criticism following the US Open champion's early exit from the Miami Open.

Raducanu was beaten by Katerina Siniakova in her first match in Miami this week after being given a bye into the second round.

She has won only four WTA Tour matches since sensationally becoming the first qualifier – male or female – to win a major in the Open Era at Flushing Meadows in September.

The 19-year-old has been in demand off the court, having last week announced she will be a brand ambassador for Porsche, but her business activities have attracted criticism.
 
Speaking after Raducanu's defeat to Siniakova, former world number five Daniela Hantuchova claimed the Briton has lost the locker room respect she had previously built up.

Kim Clijsters took aim at those who act as though they have "made it", meanwhile, though the four-time major winner did not mention Raducanu by name when making those comments.

Raducanu defended herself from the "unfair" accusations and Kyrgios has now questioned why former players have felt the need to take aim at the youngster.

"What’s with old retired players giving their opinion on our stars now?" he posted on Twitter, referencing a video uploaded by Andy Roddick on how players can curtail their anger.

"I love A-rod and I agree we all need to chill with the rackets and all that, but geezus.

"I read an article about a past female player talking about Radacanu, no offence, but she is a far, far bigger name already."

Fifth seed Andrey Rublev has been eliminated from the Miami Open in the second round in straight sets by wild card world number 102 Nick Kyrgios on Friday.

The enigmatic Australian triumphed 6-3 6-0 in 52 minutes to clinch a third round clash with Italian Fabio Fognini, who won in three sets against Japanese Taro Daniel.

The victory marks Kyrgios' third win over a top 10 opponent in the past two tournaments, having topped both Casper Ruud and Jannik Sinner at Indian Wells earlier this month.

Rublev had won titles in Marseille and Dubai earlier this season but was not allowed to settle by Kyrgios who sent down 10-5 aces and had a first serve percentage of 80 per cent.

"I know that he's a player who relies on a bit of rhythm, so I just tried to keep the points short and sharp, just play aggressive," Kyrgios said after the match.

"I'm just happy with my performance, whether it's 7-6 in the third or something like this, I'm just happy to get through."

Second seed Alexander Zverev was made to work for victory against Croatian Borna Coric, winning 6-4 3-6 6-3 in two hours and one minute.

The win marks the 2018 Miami Open runner-up's first triumph at the event since that run to the final.

Sixth seed Ruud eased past Henri Laaksonen 6-1 6-2, while ninth seed Sinner defeated Emil Ruusuvuori 6-4 3-6 7-6 (10-8) in two hours and 40 minutes.

Several seeds were beaten included 13th seed Diego Schwartzman who went down 4-6 7-6 (7-3) 6-4 to Thanasi Kokkinakis. Seeds Lorenzo Sonego, Grigor Dimitrov, Reilly Opelka and John Isner also exited.

Gael Monfils defeated Oscar Otte 7-6 (11-9) 6-1, Pablo Carreno Busta won 6-3 6-2 over David Goffin and 10th seed Cameron Norrie won 7-6 (7-5) 6-4 over Jack Draper.

Nick Kyrgios maintained his impressive form from Indian Wells to prevail in his first-round match with Adrian Mannarino at the Miami Open.

Former world number 13 Kyrgios is ranked outside the top 100, but delivered a reminder of his quality at Indian Wells by reaching the quarter-finals.

He claimed a top-10 win over Casper Ruud before losing to Rafael Nadal in three sets, and the Australian will now face Andrey Rublev in South Florida after seeing off Mannarino in straight sets.

Kyrgios took the first in a tie-break and broke the Mannarino serve late in the second to clinch a 7-6 (7-3) 6-3 win.

There was a huge disparity in the winner count, Kyrgios hitting 33 to Mannarino's 15, and it was that extra firepower that made the difference in the closing stages.

A fine backhand winner gave Kyrgios break point, which he converted with a confident forehand at the end of an absorbing rally to take a 5-3 lead. The break was then consolidated as Kyrgios sealed his triumph with an ace down the middle.

Also into the second round is Jack Draper, who overcame Gilles Simon to claim his first ATP Masters 1000 win and set up an all-British clash with Cameron Norrie.

Brandon Nakashima will meet American compatriot Frances Tiafoe after his first main-draw win in Miami over Soonwoo Kwon.

Thanasi Kokkinakis beat Richard Gasquet in straight sets while there were also wins for David Goffin and Borna Coric.

Nick Kyrgios should face "severe" disciplinary action after a ball boy was almost struck by the Australian's smashed racket, one of the best-known coaches in tennis has said.

American Brad Gilbert, a former tour player who has worked with stars including Andre Agassi and Andy Murray, was taken aback by the tantrum from Kyrgios after a three-set defeat to Rafael Nadal in Indian Wells.

Kyrgios went to the net to shake hands with Nadal, who won 7-6 (7-0) 5-7 6-4 to reach the semi-finals.

But moments later he thrashed his racket against the ground. It bounced up high, travelling half the length of the court and causing a ball boy to take evasive action, stepping out of the way to avoid being hit and potentially hurt.

Kyrgios scornfully answered reporters' questions about the incident after the match, describing it as "a complete accident".

However, the 26-year-old unmistakably lost control in front of a full stadium and could face punishment as a result.

Gilbert wrote on Twitter: "Ridiculous to do that after such a good match, the penalty should be severe."

Gilbert questioned whether that would happen, given the ATP, which runs the men's tour, only handed a suspended eight-week ban to Alexander Zverev, plus a fine, after the world number three violently hit his racket against an umpire's chair.

"Unfortunately ATP lost the plot with Zverev situation and not a good look for kids and our sport," Gilbert wrote.

The 60-year-old Gilbert, who also coached Andy Roddick, said there was "absolutely no reason to snap on court after the match".

He added: "The stick could have easily gone anywhere like the stands and hit someone, seriously awful to see that in front of great crowd."

Kyrgios posted an exchange of text messages with the ball boy, and said he would give him a racket as a token of apology.

Saying sorry for the incident, Kyrgios wrote: "Didn't want it going anywhere near you to be honest."

After his elimination from the Indian Wells Masters, Nick Kyrgios fired back at a reporter who questioned him about an incident after the match where he launched his racket, almost hitting a ball-kid.

Rafael Nadal ultimately emerged victorious in the match 7-6 (7-0) 5-7 6-4, after shaking hands with Nadal and the match umpire, Kyrgios spiked his racket into the ground as he walked back to his bench.

Speaking with post-match media, Kyrgios was sarcastic and rude when questioned about it.

"That's a question you're going to say after a three-hour battle against Nadal – that's what you've come here with?" he said.

"What would you like me to say about it? Obviously was that my intention? No, because did I throw the racquet anywhere near him originally? It landed a metre from my foot and skidded and nearly hit him. I'm human. 

"Things happen like that obviously, it was a very misfortunate [sic] bounce. What do you want me to say? It was three metres away from the kid. 

After his elimination from the Indian Wells Masters, Nick Kyrgios fired back at a reporter who questioned him about an incident after the match where he launched his racket, almost hitting a ball-kid.

Rafael Nadal ultimately emerged victorious in the match 7-6 (7-0) 5-7 6-4, after shaking hands with Nadal and the match umpire, Kyrgios spiked his racket into the ground as he walked back to his bench.

Speaking with post-match media, Kyrgios was sarcastic and rude when questioned about it.

"That's a question you're going to say after a three-hour battle against Nadal – that's what you've come here with?" he said.

"What would you like me to say about it? 

"Obviously was that my intention? No, because did I throw the racquet anywhere near him originally? It landed a metre from my foot and skidded and nearly hit him. I'm human. 

"Things happen like that obviously, it was a very misfortunate bounce. 

"What do you want me to say? It was three metres away from the kid.