Rafael Nadal believes shot clocks would ruin entertainment factor in tennis

The US Open is the first grand slam tournament to trial the innovation, with a clock on the scoreboard counting down the time taken by players between points during qualifying.

It is not being used in the main tournament, where players exceeding the limit is much more of an issue.

Nadal fully accepts he is one of the slower players, and he receives frequent warnings, but the apparently arbitrary nature of them has been a frustration both for those players penalised and opponents who feel they are not punished enough.

The Spaniard, though, thinks policing it with a shot clock would make the titanic matches he has played at the slams against Novak Djokovic in particular, impossible.

He said: "It depends on what the fans want. If the fans want short points and players playing without thinking, maybe it is good.

"If you want to have matches like I played here with Novak, the three finals, the kind of match that the crowd is more involved in because the points are so long, well, you cannot expect to play 50-shot rallies and in 25 seconds be ready to play the next tennis point.

"I think that's not possible for a great show. But if you don't want a great show, of course it's a great improvement."

Nadal goes into a grand slam as the top seed for the first time since the 2014 French Open having overtaken Andy Murray to reclaim the world number one ranking this week.

The 31-year-old is a two-time champion at Flushing Meadows but has not been beyond the fourth round since his second title in 2013 and has suffered defeats to Denis Shapovalov and Nick Kyrgios in recent weeks.

Nadal said: "It' s good to be number one of the world again, of course. I think it's a good achievement after three years having some troubles. So I'm just happy for that.

"I'm excited to start the US Open. It doesn't matter if you are seeded one or seeded 20. For me it's always special to play here in a tournament that I had success, a tournament that I enjoyed a lot of good matches, and a tournament that normally I have a great connection with the crowd."

A lopsided draw was made even more so with the withdrawal of Murray - making it five of the top 11 players absent through injury.

While Nadal and Roger Federer are both in the top half and seeded to meet in the semi-finals, Alexander Zverev is the highest seed in the bottom half.

Players in the top half can boast 54 grand slam final appearances between them (51 of them for Nadal and Federer) compared to just four in the bottom half.

Fifth seed Marin Cilic, who accounts for half of those after winning the title in New York in 2014 and reaching the Wimbledon final last month, has taken Murray's place at the bottom of the draw.

But most of the intrigue centres around fourth seed Zverev, who at 20 appears to be clearly the coming man of tennis but who is yet to have a grand slam breakthrough.

The German has already won five titles this year, two of them at the ATP Tour's elite Masters level.

The second of those came two weeks ago in Montreal, when he took advantage of Federer's back problem in the final.

Zverev does not lack confidence and is a man in a hurry, saying after his fourth-round loss to Milos Raonic at Wimbledon: "I'm so tired of learning."

The German knows this is an opportunity, and said: " I played great on the hard courts so far, won two tournaments, Montreal and Washington back to back, which are two huge events. I felt like I could compete and beat anyone.

"I feel different about this grand slam than I have felt before about the grand slams. O bviously Roger and Rafa are the biggest favourites still. But I'm just going to go match by match and hopefully I can get to those guys.

"I have gone deep with them in previous grand slams. I have beaten them in three-set matches. I have beaten them in Masters events. Obviously it will be a much longer match, but it's not dissimilar. The tennis doesn't change that much.

"Beating one of the big, big names in tennis in a five-set match, that's, I think, where the success will come in."