Andrey Rublev battles past Gianluigi Quinzi in NextGen ATP Finals

Top seed Andrey Rublev was given a scare by Italian wildcard Gianluigi Quinzi before closing out a hard-earned victory in his opening round-robin match at the NextGen ATP Finals in Milan.

Sunday's draw ceremony for the innovative tournament - which includes the first-to-four games in each set, best-of-five-set matches, no-let rule on serves and communication allowed with coaches through headsets - was criticised after featuring male players selecting female models to escort them down a catwalk.

After an upset in the opening fixture of Group B fixtures when number two seed Karen Khachanov was beaten 3-1 by fellow Russian Daniil Medvedev, it looked as if world number 37 Rublev could also slip up against Quinzi, who came through the qualifying tournament.

Quinzi - a former Junior Wimbledon champion and world number one in 2013, but currently ranked 306 on the ATP seniors circuit - stormed through the opening set 4-1.

Rublev, who reached the quarter-finals of the US Open, recovered to win the second set 4-0 in 15 minutes and then moved ahead after securing the third set tie-break 4-3 (7/3).

However, left-hander Quinzi made sure the Fiera Milano crowd, who were allowed to move around during the match, got full value by forcing a decider after an early break in set four, which he went on to take to love.

Eventually Rublev, winner of his first ATP title at the Umag Open in Croatia, came through 4-3 (7/3) in the tie-break, Quinzi having twice saved match point.

Earlier, Medvedev, seeded seventh, had pulled off an unexpected victory over Khachanov, the world number 45, when he came from behind to win 2-4 4-3 (8/6) 4-3 (7/3) 4-2.

In Tuesday's other Group B fixture, fourth seed Borna Coric of Croatia coasted past Jared Donaldson in straight sets, defeating the American 4-3 (7/2) 4-1 4-3 (7/5).

Elsewhere in Group A, Hyeon Chung of South Korea recovered from losing the opening set to beat 18-year-old Canadian third seed Denis Shapovalov, the youngest player in the eight-strong field, 1-4 4-3 (7/5) 4-3 (7/4) 4-1.