Melbourne, Australia - For the first time in four decades, there will be no American man in the fourth round of the Australian Open. John McEnroe thinks he knows why.

The seven-time Grand Slam champion, who is working as a TV analyst at Melbourne Park, suspects a lack of passion.

"You could certainly wonder whether our guys are as hungry as some of the others," McEnroe said on Fox after John Isner lost to 18th-seeded Feliciano Lopez in the third round Friday. "It really comes down to the individual. If people have heart or desire, that to me is more important than any shot."

This is the first time no U.S. player reached the men's fourth round at the Australian Open since 1973 - when no Americans traveled to the tournament.

Isner, the last American man left in the singles draw, lost to Lopez 6-3, 6-7 (3), 6-4, 6-7 (0), 6-1. It was the 6-foot-9 Isner's second straight five-set match after he knocked out David Nalbandian in a 4-hour, 41-minute thriller.

"It's very ugly, to be honest, to have no one in the round of 16," Isner said. "We've got to try to rectify that next time the big tournaments roll around.

"It's very disappointing. That's not a good effort from the Americans in this tournament. I knew going in today I was the last one left and I wanted to keep on going, but just didn't happen."

Instead, Lopez earned a fourth-round match against Rafael Nadal, who won the 2009 Australian Open and has 10 major titles to his credit.

The last American man to win the Australian Open was Andre Agassi in 2003. No. U.S. man has won a major since Andy Roddick at the 2003 U.S. Open.

"We've been spoiled with the success we've had in the past," McEnroe said.

At least the Americans still have Serena Williams, the 13-time Grand Slam champion who is on a 16-match winning streak at Melbourne Park. She won titles in 2009 and 2010 but missed last year because of injury. On Saturday night, the five-time champion will play Greta Arn of Hungary in the third round.

Vania King, the only other American in the Round of 16, lost 6-3, 6-4 to former French Open champion Ana Ivanovic on Saturday at Margaret Court Arena.

Ivanovic next plays Wimbledon champion Petra Kvitova, who was on court for 38 minutes before advancing to the fourth round. The second-seeded Czech was leading 6-0, 1-0 on Rod Laver Arena when Maria Kirilenko retired with a left thigh injury.

It was the fourth retirement of the women's tournament.

Maria Sharapova was tested for the first time and still came out with a 6-1, 6-2 win over Germany's Angelique Kerber.

The 2008 champion has lost five games in reaching the fourth round at Melbourne Park. She won her first two matches 6-0, 6-1 despite not playing any warmup tournaments because of an ankle injury.

The scoreline on Saturday made it look easy enough for Sharapova, but she was pushed during a 56-minute second set, with many games going to deuce.

"She certainly stepped up in the second set," Sharapova said. "She reached the semifinals at the U.S. Open last year so she's been on the big stage before and I knew she could produce some really good tennis."

Former finalist Jo-Wilfried Tsonga hardly broke a sweat during a commanding 6-2, 6-2, 6-2 win over Frederico Gil of Portugal. He was one of six French men who made the third round.

When four American men reached the fourth round of the U.S. Open in September - only two majors after no Americans reached the round of 16 at the French Open - Roddick talked about a "healthy jealousy" contributing to the improvement. There hadn't been four American men into the fourth round at the U.S. Open since 1995.

It didn't last long in Melbourne. Among the best contenders, No. 8 Mardy Fish lost to Alejandro Falla of Colombia in the second round before Roddick hurt his right hamstring and had to retire from his second-round match with former No. 1 Lleyton Hewitt.

Ryan Harrison took No. 4 Andy Murray to four sets before losing; Ryan Sweeting lost to No. 5 David Ferrer; and Donald Young lost to qualifier Lukas Lacko.

"People expect us to contend for Grand Slams," McEnroe said. "I think there's a lot of tennis fans, but to have an American contending and winning majors, that would make a big difference."

The pressure of being last man standing got to Isner on Friday when he lapsed in the last set after dominating the fourth-set tiebreaker.

"It just got away from me. I just wasn't as sharp as I needed to be," he said. "It just kind of spiraled out of control there, and it just ... I couldn't climb out of the hole I dug."

"I honestly felt like it was more mental than anything," he added. "Just like the whole match I wasn't really that sharp."

Now he has to consider a Davis Cup series next month against a Swiss team that could feature Roger Federer and Stanislas Wawrinka.

"That's not an ideal draw for a first round. But, yeah, it's going to be tough," Isner said. "It's away, and, you know, more than likely I think if Roger plays we're probably the underdogs."

Top-ranked Novak Djokovic started 2011 on a 41-match winning streak and finished it with the No. 1 ranking and three of the four major titles. His two main rivals are already through to the third round on the other side of the draw.

Nadal had a 6-2, 6-4, 6-2 win over Lacko, the last qualifier in the draw. Apart from having his right knee heavily taped, he showed no sign of pain from the bizarre injury - he hurt his knee while sitting in a chair at his hotel - that made him think he might miss this tournament.

"The knee is fine ... being in the fourth round without losing a set, it's fantastic news," he said.

Federer followed with an almost flawless performance in a 7-6 (6), 7-5, 6-3 win over 6-foot-10 Croatian Ivo Karlovic. Federer will play Australian teenager Bernard Tomic on Sunday.

Tomic, a Wimbledon quarterfinalist last year, had a tough 4-6, 7-6 (0), 7-6 (6), 2-6, 6-3 win over 13th-seeded Alexandr Dolgopolov of Ukraine.

An early rematch of last year's women's final is already in place, with defending champion Kim Clijsters and Li Na both winning Friday night to set up a meeting in the fourth round.

Clijsters advanced with a 6-3, 6-2 win over Daniela Hantuchova. Li didn't even finish four games before Anabel Medina Garrigues quit with a badly sprained right ankle.

Medina Garrigues twisted her right ankle in the second game and needed treatment on the court. Li won the first three games and had taken the first two points in the fourth game on Medina Garrigues' serve when the Spaniard hit the ball into the air with her racket. She then went - in tears - to the net to retire.

The winner of the Clijsters-Li match will likely face a quarterfinal against top-seeded Caroline Wozniacki, who has not dropped a set in advancing to the fourth round as she continues her quest for a first Grand Slam title. She beat Monica Niculescu of Romania 6-2, 6-2, while third-seeded Victoria Azarenka defeated Mona Barthel 6-2, 6-4.

Wozniacki will next play former No. 1 Jelena Jankovic, who beat Christina McHale of the U.S. 6-2, 6-0.