Novak Djokovic became the first player to qualify for the semi-finals of the 2018 ATP World Tour Finals after Wednesday's action at the O2 Arena in London.
The Serb saw off third seed Alexander Zverev in straight sets to secure his second group win in the early encounter.
This year's tournament then saw its first three-set match, when fifth seed Marin Cilic came from a set down to beat the United States' John Isner.
Wednesday's Results, Group Guga Kuerten
(1) Novak Djokovic bt. (3) Alexander Zverev 6-4, 6-1
(5) Marin Cilic bt. (8) John Isner 6-7 (2), 6-3, 6-4
Thursday's Schedule, Group Lleyton Hewitt
Not before 2 p.m. GMT/10 a.m. ET: (6) Dominic Thiem vs. (7) Kei Nishikori
Not before 8 p.m. GMT/3 p.m. ET: (2) Roger Federer vs. (4) Kevin Anderson
Djokovic ultimately ran out a comfortable winner against Zverev, but he was made to work hard to claim a close first set.
Zverev had two chances to break with Djokovic serving at 4-4. He missed the first with an overhit return, but a double-fault from the top seed offered him another opportunity.
Again Zverev went long with a lob, allowing Djokovic to hold for a crucial 5-4 lead. The Serb forced three break points in the next game. Zverev saved the first but double-faulted to hand his opponent the first set.
Sports writer Tumaini Carayol offered his view:
Zverev managed to stay in contention at the start of the second set. However, a series of errors allowed Djokovic to break for a 3-1 lead, and he didn't look back on his way to victory.
The Serb raced away with the second set to complete the win in 76 minutes.
Zverev spoke about how he was feeling after the match:
It was another commanding win from Djokovic, after his opening victory over Isner on Monday. He remains the hot favourite to win in London and will complete his group stage against Cilic on Friday.
Wednesday's second match saw big servers Isner and Cilic meet, and it was little surprise to see the first set go all the way to the tiebreak.
The American did have chances to break at 5-4 and clinch the set, but Cilic managed to hold serve with some aggressive play.
Isner took the initiative in the tiebreak and was rewarded with the opening set. Some powerful forehand hitting proved too much for Cilic and was also good news for Djokovic:
Both players had chances to break at the start of the second set. Cilic earned two break points at 3-2 with a wonderful forehand, followed by a brilliant backhand return. Isner responded with three aces in a row to hold.
Cilic kept up the pressure and earned three more break points at 4-3. Isner saved the first two, but a first double-fault of the match gave the Croat his first break. He served the second set out at love to send the match into a decider.
The initiative was with the fifth seed at the start of the third set. Cilic's returning was causing Isner problems, and another double-fault gave him an early break and prompted an angry reaction from the American.
Yet Cilic lost the advantage immediately. A sloppy game allowed Isner to break straight back and make it 1-1, before he dropped serve again and allowed Cilic back into the lead.
This time Cilic did manage to keep his concentration, as he claimed just his second win in 11 ATP Finals matches.
Both players can still qualify for the semi-finals and are back in action on Friday:
Cilic will face Djokovic again having met him earlier in November in the quarter-finals of the 2018 Paris Masters. The world No. 1 came from a set down to win in three sets against the Croat.