Novak Djokovic made it three wins out of three at the Barclays ATP World Tour Finals to qualify for the semi-finals at London's O2 Arena.
The world number one was in impressive form as he saw off Tomas Berdych 6-2 7-6 (8/6), saving three set points in the second set.
Djokovic is now guaranteed to top Group A while Berdych is out, and Andy Murray only needs a set against Jo-Wilfried Tsonga to ensure he will also qualify for the last four.
Berdych had beaten Djokovic only once in 11 previous meetings, in the semi-finals of Wimbledon in 2010, and he was soon under pressure. Djokovic's famed returns were rushing the Czech and, although he clawed his way back from 0-40 in the third game, a fourth chance arrived and his opponent took it.
Djokovic, whose best performance at the O2 was a semi-final appearance in 2010, looked extremely sharp and he made it five games in a row before Berdych finally stopped the rot. The top seed comfortably held to win the set, though, and with that assured himself qualification for the last four.
Berdych appeared to have no answer when Djokovic broke again in the third game of the second set but he began to take the fight to his opponent a bit more and hit back to level at 2-2. The crowd were keen to see a good match and got behind the Czech, who saved more break points to edge ahead in the set.
Berdych was beginning to match Djokovic from the back of the court and making his extra power count, and he made a flying start to the tie-break, winning five of the first six points.
But Djokovic did not give up and saved two set points on his own serve before another trademark return at Berdych's feet levelled matters at 6-6. The top seed then added a fourth point in a row and clinched victory when Berdych fired a return long.
Djokovic, who is now likely to avoid defending champion Roger Federer in the semi-finals, has built a reputation for clawing his way back from difficult positions in sets and matches.
He said: "I just try to have simple thinking, to fight for every point. Sometimes it doesn't work. I'm glad it worked. For the first time I won all three matches in my group. This is very encouraging for me, so I will try to focus on the semi-final now."