Tendulkar, who only needed 25 runs to reach the landmark at the start of his innings, was 57 not out at stumps on the second day of the second Test against England with India, at 254 for three, 56 runs ahead on first innings.
Former South Africa quick Donald, now England's bowling coach, said of Tendulkar: "He's the best player I've ever had the pleasure of playing against. He's something special, very freakish.
"I think not only what he's done for the game but what he's done for his country has been absolutely phenomenal. He's just a credit to India and james anderson, bad trot, injuries, old self, grilled, sconned, cricket."
Tendulkar did have an uncomfortable moment early in his innings when, on five, he was hit on the helmet as he tried to sway out of the way of a short ball from James Anderson.
"I know he's been through a bad trot with injuries over the last couple of years but he looked quite his old self out there apart from being 'grilled' today. It's not many times you see him getting 'sconned'," added Donald.
"He's definitely the best I've ever seen."
Now only the retired duo of world record holder Brian Lara, with 11,953 runs from 131 matches and former Australia captain Allan Border (11,174 from 156) are ahead of Tendulkar in the list of Test cricket's all-time leading run-makers.
Donald, asked if the 34-year-old Tendulkar, appearing in his 139th Test, could break Lara's record replied: "I hope so. There's still a lot more cricket in Sachin Tendulkar. He still looks so boyish, it's just unbelievable how many years he's stuck it out and has achieved what he's achieved."
Tendulkar, who made his Test debut as a teenager in 1989 against Pakistan, was the first batsman to score 50 hundreds in all international cricket.
He is also the leading century maker in Test history, with 37, and recently became the first man to score 15,000 one-day international runs.
"It's scary to see how many hundreds he's clocked up in Test matches, not to mention one-day internationals," said Donald. "I hope he goes past it because I think he deserves it simply because he's such a humble, wonderful guy."
India opener Dinesh Karthik, who made 77 on Saturday and put on a valuable 147 for the first wicket with Wasim Jaffer, showed that even Tendulkar's team-mates were just as admiring of the 'Little Master' as outsiders.
"For me he's the greatest player I've ever seen," said the 22-year-old, appearing in his 15th Test. "I'm humbled to be part of the same team and I'm honoured that a person like Sachin even knows my name.
"I'm 100 per cent sure he will break the record. It's just a matter of time."