Andy Murray

The 35-year-old two times Wimbledon champion, a former world number one currently ranked 52nd, bowed out in the second round after a four set defeat to big-serving American John Isner, the 20th seed.

"I could have had a good run here," he said. "One of the reasons why improving your ranking and trying to get seeded is important, (is to) avoid playing top players and dangerous guys like that early in tournaments.

"It's one of those matches that, had I got through, who knows what would have happened," added the Scot. "It's frustrating for different reasons."

Players from the two countries cannot play at this year's Grand Slam because of Russia's invasion of Ukraine.

The All England Lawn Tennis Club (AELTC) has also banned Russian and Belarusian players from all other UK grass-court tournaments.

Meanwhile, Rafael Nadal called the ban "very unfair".

Belarusian players were included in the ban because the country has supported Russia's military action in Ukraine.

The former world number one is working with Unicef to provide medical supplies and development kits.

Russia's attack on Ukraine has so far seen more than two million people flee the country.

The three-time Grand Slam winner, who is an ambassador for Unicef UK, said: "It's vital education continues, so Unicef is working to enable access to learning for displaced children, as well as supporting the rehabilitation of damaged schools, together with replacement equipment and furniture."

Latvia's Jelena Ostapenko took home $154,000 after winning the women's WTA 500 event by beating Veronika Kudermetova in the final last weekend.

The men's ATP 500 event, which ends this weekend, will result in the winner receiving prize money worth $773,000.

"Obviously that's a big step backwards, so it's not great," Murray said after his 7-5 6-2 loss to Jannik Sinner.

"Obviously if they're the same size events one week apart, that's quite a big discrepancy."

The 34-year-old Murray, who crashed out in the second round of the Australian Open, will be in action at the ATP 500 event in Rotterdam this week after accepting a wildcard to enter the main draw.

"The past couple of years, the clay has made issues worse," the three-times major winner told reporters ahead of his first-round match against Kazakhstan's Alexander Bublik.

"Last year I had some issues at the beginning of the year, the clay didn't help, so I've spoken to my team about that and this year while I feel good and healthy, I don't want to take that risk."

World number one Murray struggled with the issue prior to his Wimbledon defence and appeared to be suffering the effects when he was beaten in the quarter-finals by Sam Querrey.

The Brit has not played since that match and pulled out of the Rogers Cup, meaning he could well be replaced at the top of the rankings by Rafael Nadal this month.

However, Murray remains hopeful of being in condition to compete at the US Open.

Andy Murray defended his men's singles title at the Olympics, overcoming Juan Martin del Potro in a thrilling final at Rio 2016 on Sunday.

The Brit became the first player to win back-to-back singles gold medals thanks to his 7-5 4-6 6-2 7-5 victory at a loud Olympic Tennis Centre on Sunday.