HOUSTON -- Andre Agassi paid quite a compliment to Roger Federer: He likened the reigning Wimbledon champion's on-court grace and cool to Pete Sampras'.

Those qualities came in handy for Federer in his opening round-robin match at the Tennis Masters Cup, facing a deficit against Agassi and a partisan crowd.

Federer played through his rough patches Monday night and erased two match points to get past Agassi 6-7 (3), 6-3, 7-6 (7), keeping alive an outside chance of ending the year with the No. 1 ranking.

"His disposition and the way he moves on the court -- you never feel like he's out of control. You never feel like he's panicked," Agassi said. "While any athlete knows that you can't help but feel these things, he does a great job of keeping that gracefulness about him."

Federer pretty much ceded the first set Monday, with three straight forehand errors helping Agassi take a 6-0 lead in that tiebreaker. But the third-ranked Federer kept going for lines with his serves and ground strokes, compiling 20 aces and a total of 63 aces -- overcoming 50 unforced errors.

"I knew that if I just didn't freak out mentally, my forehand would come back eventually," Federer said.

He was right.

Federer -- at 22, he's 11 years younger than Agassi -- had fresher legs and crisper shots in the last tiebreaker. He did away with Agassi's second match point at 7-6 with a runaround forehand winner and ended the nearly 2{-hour match by smacking a cross-court forehand passing winner on a full sprint.

If he's going to catch current No. 1 Andy Roddick, Federer needs to win another four straight matches, through the round-robin portion, the semifinals and final.

Roddick's first match at the season-closing event is Tuesday night against No. 7 Carlos Moya. The other player with a chance at topping the rankings is No. 2 Juan Carlos Ferrero, who lost to No. 8 David Nalbandian 6-3, 6-1 on Monday.

In May, Agassi became the oldest man to reach No. 1 -- he's now No. 5 -- and his 58 career titles match the total of the seven other Tennis Masters Cup participants.

But he couldn't muster his absolute best in the clutch against Federer. In the final tiebreaker, Agassi had a double-fault, dumped one easy forehand into the net and wasted a match point with a poor return.

"Those are just the signs of not playing in a few months," Agassi said.

His last competitive match was Sept. 6, a loss to Ferrero in the U.S. Open semifinals. Steffi Graf, his wife, gave birth to their second child, daughter Jaz, in early October.

It also didn't help Agassi's cause that Federer hit several spectacular shots. On two, he essentially volleyed from the baseline, swinging away for stinging winners without letting Agassi's shot hit the purple hard court.

"I just believe in taking chances," Federer said. "Sometimes it works."

Agassi took fewer chances, finishing with 22 winners and 28 unforced errors.

And one of the greatest returners in the history of tennis was at times befuddled by Federer's serves, which aren't the fastest around but are quite effective.

"He never misses the serve. If he misses it, it's just out. But it's never sitting in your strike zone, because he's always putting it close to the line," Agassi said. "Like a good pitcher who's working the outside of the plate."

Their match felt like a Davis Cup final, from the U.S. flag beneath the scoreboard, to the "USA" signs, to the red, white and blue body paint on dozens of fans, to the "Let's go, Andre!" cheers that began after the first point.

By the second set, the pro-Agassi chanting was so incessant that even other partisan spectators in the sellout crowd of 7,500 responded with, "Sssshhhhh!"

"I wasn't too surprised. Americans always scream a lot. Less clapping, more screaming," Federer said. "That's OK. All crowds should be different."