Rafael Nadal revealed he is being affected by pain in his back after reaching the fourth round of the French Open.

The eight-time champion and world number one has not yet dropped a set and, a sticky patch in the second set aside, was pretty much untroubled in a 6-2 7-5 6-2 victory over Leonardo Mayer on Saturday.

Nadal was struck down by back problems in the Australian Open final in January, which he lost to Stan Wawrinka.

It appeared it had taken his mind longer to recover from the disappointment than his back but strapping was visible during his victory over Mayer.

Nadal said: "I felt a little bit my back, so that's why I slowed down a little bit the serve. I felt it a little bit from the beginning (of the tournament).

"In the second match I was not serving that fast, too. I had the problem in Australia. It's real. That's the real thing. But hopefully (it is) not (a concern)."

Nadal won five of the first six games and looked on his way to a very straightforward victory when he broke the Mayer serve again early in the second set.

But Mayer began to put a lot more pressure on the Spaniard and got his reward when he broke back for 4-4.

In the next game he was two points away from taking the set but Nadal held on and then broke the Mayer serve once more with a precision lob, the Argentinian imploring the ball to go long then shaking his head when it landed on the line.

Nadal let a break slip away at the start of the third set as well but finally broke Mayer's resistance with a run of five straight games to win the match.

The Spaniard said of his week: "It was a positive one.

"I have three matches that I think I played under control. That's always positive and good. Always to be in the second week is great news, and winning in straight sets is a little bit better. I'm happy for that."

Next the world number one will face Serbian Dusan Lajovic, who is through to the last 16 on his first appearance at Roland Garros.

The 23-year-old, who defeated Jack Sock in straight sets, said: "I can't be more happy than this. To play Rafa on Chatrier is one of the biggest dreams and biggest challenges."

The much-anticipated clash between Gael Monfils and Fabio Fognini was a predictably wacky five-set affair.

With two of the most flamboyant players meeting, it was a match where anything was possible, and it was Monfils who eventually came out on top 5-7 6-2 6-4 0-6 6-2.

The pair made 137 unforced errors between them and Monfils recovered to win despite appearing barely able to move in the fourth set.

The Italian, meanwhile, was given a point penalty in the third game of the deciding set for throwing his racquet having previously received a warning for swearing.

Monfils is through to the fourth round at a grand slam for the first time since reaching the quarter-finals at Roland Garros three years ago and will play Spain's Guillermo Garcia-Lopez.

Explaining his fourth-set dip, Monfils said: "When I started in the fourth, he broke me straight away. I was starting to struggle a little bit, not cramping, but I felt really tired.

"I tried to break back, I couldn't break back. I wanted to serve first in the fifth. The only option I had is to take 6?0. Why should I make any effort?"

Fifth seed David Ferrer is yet to drop a set this tournament, with 32nd seed Andreas Seppi his latest victim in a 6-2 7-6 (7/2) 6-3 victory.

Last year's runner-up has now made at least the fourth round in 16 consecutive grand slams and will look to make it 10 straight quarter-finals when he takes on Kevin Anderson.

The 6ft 8in South African was involved in a battle of the giants against 6ft 11in Ivo Karlovic but the Croatian had to pull out after a set because of a back problem.