Despite the myriad achievements in his career, one moment stands out for Federer as the best match he's ever played.
Federer said he understands well how even a champion like Djokovic could be struggling to find his game so soon after having surgery.
He is sure to face a tougher task against Australian Open semi-finalist and South Korean star Hyeon Chung, who rallied past Dusan Lajovic 6-7 (9-11) 6-3 6-3.
Djokovic scrambled to win the second set on Sunday but the rust from the layoff was evident as he made 62 unforced errors overall.
"When you do come back from injury or when you haven't played in a long time, it just takes extra effort". "I'd like to congratulate my opponent for fighting and showing great, great spirit on the court", said Djokovic. "I mean, I just completely lost rhythm, everything".
"Look, still such early stages for Novak coming back and the first one after surgery".
Thiem has already won a tour-leading 17 matches this season, but he had to work hard to beat 19-year-old Tsitsipas, who reached the quarter-finals in Dubai last week and is ranked 71st in the world.
In-form seventh seed Kevin Anderson accounted for Evgeny Donskoy 7-5 6-4.