Roger Federer continued his serene progress at the US Open on Saturday to remain on course for a showdown with the man who ended his amazing Grand Slam streak.
Federer overcame strong winds to beat France's Paul-Henri Mathieu 6-4 6-3 6-3 in one hour and 39 minutes and is yet to drop a set in three matches at Flushing Meadows.
The five-time champion is looking to reach his seventh straight final in New York and regain the title after losing to Argentina's Juan Martin Del Potro 12 months ago.
But he looks set to face Sweden's Robin Soderling in the quarter-finals, the player he lost to at the same stage of the French Open this year to end his streak of 23 consecutive Grand Slam semi-final appearances.
Speaking about his win over Mathieu, Federer said: "I thought the first set was the key. He has some serious talent. We came through the juniors together and he used to toy around with me.
"I got a bit lucky in the first set, he had the first break point at 4-3 and after that I was able to break him back and maybe break his will a bit."
Federer announced before the championships began that he has hired Paul Annacone as his coach full time after a successful trial period and added: "He's a good guy and we have known each other a long time since he was working with Pete [Sampras] and Tim [Henman].
"We had a good test after Wimbledon, he's making me play a little more aggressive and it's good to hear from someone new. I haven't lost many matches on the hard-court season."
Soderling needed five sets to edge past Austrian qualifier Andreas Haider-Maurer in the first round, but has since dropped just 16 games, the fifth seed beating Thiemo De Bakker 6-2 6-3 6-3 on Saturday in just an hour and 42 minutes.
"Everybody has been telling me since the draw came out I'm going against Roger in the quarters," Soderling said. "It's still far ahead.
"I'm the fifth seed, so I know I'm gonna play one of the four top guys in quarters. They're all good players so it doesn't really matter who I'm playing.
"I started my first match maybe not so great, but after that I think I played two pretty good matches. And today was not easy. It was difficult conditions. I told myself before the match, 'It's not gonna be pretty.'
"I think, given the circumstances, I played a pretty good match."
Montanes had earlier become the first man into the last 16 after Japan's Kei Nishikori was forced to retire from their third round match.
Montanes, the 21st seed, had won the first set 6-2 and was leading 2-1 in the second when Nishikori indicated he was unable to continue with a groin injury.
Nishikori, who came through three matches in qualifying just to reach the main draw, was possibly suffering the effects of his second round win over 11th seed Marin Cilic.
The 20-year-old beat Cilic 5-7 7-6 3-6 7-6 6-1 in a match lasting just a minute shy of five hours.
France's Gael Monfils also advanced to the last 16, the 17th seed beating Serbia's Janko Tipsarevic in four sets to set up an all-French clash with Richard Gasquet, who beat South African Kevin Anderson 6-4 7-6 (7/3) 7-5.