Big-serving Milos Raonic next in firing line for Murray

The emerging 'Maple Leaf Missile' offers different challenge for the British No 1

Andy Murray feels the heat during his win over Feliciano Lopez on Saturday (AP)

After beating Feliciano Lopez 7-6, 7-6, 4-6, 7-6 in nearly four hours at the US Open on Saturday Andy Murray admitted that playing a big server like the 31-year-old Spaniard could be "mentally tough because you can have games when you're not touching the ball".

Just as a major workout in gruelling heat and humidity should have helped Murray prepare for potentially tougher challenges later in the tournament, so the experience of returning Lopez's thunderbolts could be useful when the Scot meets Milos Raonic, the world No 16, in the fourth round here today.

Lopez's fastest serve was recorded at 136mph (Murray's was 133mph). Raonic reached 143mph in his straight-sets victory over James Blake, which was still well short of the 6ft 5in Canadian's best. In San Jose earlier this year he hit a 155mph serve, which put him third on the all-time list, tied with Andy Roddick and behind only Sam Groth and Ivo Karlovic.

Nevertheless, Murray will know that the 21-year-old is by no means a one-trick pony. Raonic beat him in their only previous meeting in Barcelona earlier this year – on a clay court, which would only have slowed down his serve – and is regarded by most people in tennis as the best of the next generation behind Murray.

"He's obviously playing better and better," Murray said. "He's gaining experience all the time and this is his best year on the tour so far. He's definitely going to be dangerous. He's improved a lot from the back of the court."

Raonic said: "I think if it was an issue of me just being able to serve, a lot of opponents would feel no pressure just blocking the return back in the court. I'm able to create more after the serve. Instead of them just feeling the freedom to block the ball in, they feel they have to do a bit more.

"I'm improving all the time. I'm working on it. It's a big part of the game I focus on, as well as the serve. I know for me to keep improving up the rankings, as a player, as a competitor, I'm going to have to keep improving there as well as my serve."

The serve, nevertheless, is the weapon that makes the "Maple Leaf Missile" particularly dangerous, especially on the fast courts and in the quick conditions here at Flushing Meadows.

"He has a huge serve," Murray said. "He goes for his second serve as well. He can serve some doubles, but also get free points from his second serve, too. It's a similar kind of match to playing [John] Isner. He probably can't hit the spots that [Isner] can because of the height, but is maybe a little bit more solid from the back of the court. It will be tough."

Raonic has won three titles already but has never gone beyond the fourth round of a Grand Slam tournament, although this is only his eighth appearance at this level. This year he lost to Lleyton Hewitt in the third round of the Australian Open, to Juan Monaco at the same stage of the French Open and to Sam Querrey in the second round at Wimbledon. In the first round here he was trailing by two sets to one and 3-1 down in the fourth set against Santiago Giraldo.

Murray will be happy that lower temperatures are forecast. His match against Lopez was played in the hottest part of the day under a blazing sun, in 33C heat and high humidity. Until he got a second wind in the final set Murray was struggling. He looked tired and did not move well, stumbling to the floor on at least three occasions. The Scot normally has a post-Wimbledon training block in Miami, to help him to adjust, but that was not possible this summer because of the Olympics.

"It does affect your movement a little bit," Murray said when asked about how he had been affected by the conditions. "The court is so hot. You feel it right through your shoes so your legs get warm. Your feet are pretty sore by the end of it.

"Keeping your focus [can be tough]. Maybe if I had concentrated a little bit better today I could have finished the match in two and a half hours rather than four. But it is hard in those conditions to stay focused for a long time."