Roger Federer's number one ranking is under threat after he suffered an opening-match loss to Gilles Simon 2-6, 7-5, 6-4 in the second round of the ATP Masters Series event in Toronto on Wednesday.

After starting 2008 with a lead of 1,445 ranking points over Rafael Nadal, ten defeats in 53 matches has left Federer facing the possibility of losing the top spot he has held for 234 weeks since 2004.

"It's a bad start, no doubt," Federer said. "I wish it could have been different. Now I'm going to get some practise in."

While Nadal cannot take over the top spot even with a second title in Canada, the pressure is now huge on 12-time Grand Slam champion, and only a few hundred points will separate the leading pair if the Spaniard does well this week.

"The problem was my game today," Federer added. "As the match went on I struggled a little bit to put the forehands away. "[Simon] is a good baseliner. We saw that today. He moves well. He's deceiving because he's kind of thin and tall but moves really well for his height. He flicks a lot of balls with his backhand as well, so when you come in you can't see where he plays."

Nadal waited out a six-hour rain delay but won his 25th match in a row, beating Jesse Levine 6-4, 6-2. "Today wasn't my best match obviously, but I didn't feel very bad after the beginning," he said. "I know it's a tough tournament, and I'll have to play better tomorrow. If not, I'll have a lot of problems."

Nadal said that he must improve as he faces Russian Igor Andreev, who upset 16th seed Tomas Berdych 6-4, 3-6, 6-4.

"I have to move better than today, that's for sure," he said. "That will make everything a little bit easier. The movement is a little bit different here on hard. And I have to be more aggressive on the backhand."

Simon, 23, could not believe his success over Federer in their debut meeting when he broke the Swiss five times.

"I don't know what to think. This is just unbelievable to beat Roger like this," Simon, ranked 22nd in the world, said. "I was really tight at the beginning but then I had a nice sensation from the baseline, I played my tennis and fought on every point. I can't believe I won this match.

"I was playing like in a dream, I just saw the ball, and I hit is as hard as possible. It's so nice to play three sets like this and to win the match."

Federer is determined to fix what's wrong with his game. "It's important to stay positive. The hard court season just started and we have nine months of it," he said. "It's not the end of the world, but I wish it could have started better. I've got to regroup and look forward.

"The bigger picture is the Olympic Games and the US Open. Those are really the places where I want to win, so I have to make sure I'm ready."

Four other seeds advanced with number four Nikolay Davydenko putting out German Tommy Haas 6-3, 7-6 (8/6) and David Ferrer beating American Robby Ginepri 6-4, 6-4.

Seventh seed James Blake overhauled Swede Jonas Bjorkman 1-6, 6-1, 6-2 and Richard Gasquet earned a win for France over Andrey Kudryavtsev 7-5, 6-3.