Djokovic won the first set 6-3, and del Potro took the second 6-3. As they went to the sideline, the chair umpire announced play would stop for the day.
Tandon: Suspended streak
Will Novak Djokovic's 41-match winning streak continue? It's a cliff-hanger as his match against Juan Martin del Potro is suspended by darkness, writes Kamakshi Tandon. Story
Two-time Australian Open champion Djokovic is 39-0 in 2011 and has a 41-match winning streak dating to December. Del Potro won the 2009 U.S. Open and was a semifinalist at Roland Garros that year.
They played for 1 hour, 37 minutes before Friday's match was halted at 9:15 p.m.
When Federer's serve is clicking, he's awfully tough to beat; it's one of several skills that propelled him to his record 16 Grand Slam titles, including the 2009 French Open. He does it with guile and placement, rather than overwhelming speed, never coming close to the 140 or 150 mph that big hitters such as Andy Roddick or Ivo Karlovic can produce.
"I don't know (if) anyone ever hit more lines from first serves than he did today. And I'm not saying that it has anything to do with luck. It was just big, good, precise serves," Tipsarevic said.
"I have problems if somebody is serving like Roger, really close to the lines. I don't think he served more than five serves over (120 mph)," Tipsarevic continued, "but every serve was, like, five centimeters from the line or on the line."
Federer's fastest serve Friday was clocked at 130 mph, and he averaged 115 mph on first serves.
But it seemed as though he put nearly every ball right where he wanted to -- iIncluding when there was a loose ball bouncing near the baseline between points, and Federer swung his racket behind his back.
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"Variation has always been a key to my serve, because I'm never going to be able to serve, you know, what Roddick and all those guys can serve at," Federer said. "So I have to move it around better, hide it with the toss and so forth. I think right now it's going well."
That's probably an understatement.
During three straight-set matches this week, his opponents have accumulated a total of only two break points, converting one. Federer has 29 aces and two double-faults.
Starting from late in the second set's sixth game Friday -- when Tipsarevic held his lone break point -- Federer won 27 of the last 34 points on his serve.
"Completely different player if he's leading or if he's losing," said Tipsarevic, who is 0-4 against Federer but pushed him to 10-8 in the fifth set at the 2008 Australian Open. "If he starts leading, or starts feeling confident on his service games, it's really tough to play him, because you never know what he's going to do."
When Federer plays poorly, he'll often point to his serve as being problematic.
"Sometimes you go through days where it's easier and sometimes where it's a bit harder, and also depends on who you play," said Federer, who next faces Swiss Davis Cup teammate Stanislas Wawrinka, a five-set winner over No. 17 Jo-Wilfried Tsonga of France on Friday. "So far in this tournament, I've been doing a good job, but it's really only at the end of the tournament where I would think it really matters how I did."
He reached four finals in a row in Paris from 2005 to 2009, losing the first three to Rafael Nadal.
A year ago, though, Federer was eliminated by Robin Soderling in the quarterfinals, ending a run of 23 Grand Slam semifinals in a row.
He lost in the quarterfinals at Wimbledon, too, and then at the semifinals at the U.S. Open and Australian Open, making this his longest stretch without a major title since he earned his first at the All England Club in 2003. It's also the first Grand Slam tournament since then that Federer isn't seeded No. 1 or No. 2.
Instead, he's seeded No. 3, behind Nadal and Djokovic.
Which, Federer says, is OK. He also says he's in a better place than he was 12 months ago.
"I'm at peace with my game right now. I'm physically fine. I think I had a good preparation, so there's no reason to get nervous," he said. "I'm still in the tournament. It's always nice to advance in the draw so well, so quickly."
In other results, seventh-seeded David Ferrer also won, beating No. 31 Sergiy Stakhovsky of Ukraine 6-1, 6-1, 6-3. No. 9 Gael Monfils of France also advanced, while Albert Montanes of Spain defeated No. 12 Mikhail Youzhny of Russia 6-1, 7-6 (0), 6-1 and Fabio Fognini of Italy beat No. 30 Guillermo Garcia-Lopez of Spain 4-6, 6-3, 6-3, 6-1.
Information from The Associated Press was used in this report.
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