Don't count Roger Federer out yet. That's what Pete Sampras says, anyway, and he would know.

Although the Swiss ace - who surpassed Sampras' former record of 14 major titles on the way to 16 of his own - seems to have lost his Grand Slam form, Sampras said "it's still possible" Federer could win more.

"I feel like Roger can still win majors," Sampras said. "He's got some troubles now. But he's got a tough number (of Grand Slam titles for others) to catch up to. Nadal certainly has a chance, but physically it's not easy for him to do it."

Sampras admitted stars today have more taxing competition than during his era, and hailed Novak Djokovic as the strongest among the big four.

The Serb claimed every major but the French Open this year, becoming the sixth player to do so in the open era. He edged out Nadal and Federer to secure the year-end top ranking.

"It's hard to duplicate what he did this yearj," Sampras said. "I won't count Roger out, and Rafael will also be there. Murray is certainly getting close. But Djokovic is in the best year that I've seen in my lifetime. What he's done and whom he's beaten suggest he's in the best form we've ever seen."