The junior guard finished with 19 points but it was the way he handled the last few minutes of the game that made the difference in the 10th-ranked Musketeers' 74-66 victory over the Red Storm.
''I've come to expect the way he played down the stretch each and every game,'' Xavier coach Chris Mack said of Davis ''He's able to keep a calm cool head. He got to the rim and hit a floater. He got fouled and made his free throws. He was the closer tonight.''
Trevon Bluiett added 18 points for Xavier, which went down to the final minute before securing the win that was the Red Storm's sixth straight loss.
Jalen Reynolds had 10 points and 13 rebounds for the Musketeers (14-1, 2-1 Big East), who seemed like they had a comfortable margin several times in the second half only to see the Red Storm (7-9, 0-3) get as close as one point with 2 minutes to play.
''Despite our lead we weren't playing very hard or very well,'' Mack said.
Mussini hit a 3-pointer to bring the Red Storm, who trailed by 12 points with 6:43 to play, within 65-64 with 2 minutes to go. On the next possession, Bluiett was fouled by Durand Johnson as he attempted to rebound a missed 3 by Davis. Mussini slammed the ball to the court and was called for a technical foul. The Musketeers made three of the four free throws to lead 68-64 with 1:33 to play and they outscored St. John's 6-2 the rest of the way.
''I was upset with myself because I didn't box out and we lost the rebound,'' Mussini said. ''I didn't think about it and slammed the ball and didn't even know (the technical) was on me. I asked who it was on. Without that we probably would have won this game.''
Mvouika jumped right in when Mussini tried to take the blame.
''I don't want him to have that on his shoulders,'' he said. ''I turned it over with 30 seconds left so I can say I lost the game. It didn't go wrong right there.''
Davis scored 16 points over the final 9 minutes for Xavier, which shot 48 percent (24 for 50) and the Musketeers were 4 of 14 from 3-point range.
''Give them credit for the way they came back,'' Mack said.
Mussini, a freshman, finished 5 of 8 from 3-point range but the rest of the team was 2 for 21 from behind the arc.
''They had a pretty good lead and we battled and competed and didn't give up,'' St. John's coach Chris Mullin said. ''I tell my guys we're going to break through if we keep competing, keep practicing and focus on that. They keep doing that we're going to get rewarded.''
The teams went in different directions early with their shooting.
Xavier took a 16-4 lead with 13:41 left in the first half by making seven of its first nine shots from the field. St. John's, meanwhile, started out by missing 12 of its first 13 shots.
The Musketeers led 33-26 at halftime after shooting 52 percent (13 of 25). The Red Storm finished the half 10 of 37 (27 percent) and they were 3 of 13 from 3-point range.
The news wasn't good at halftime for St. John's. Freshman center Yankuba Sima, who leads the Big East in blocked shots at 4.1 per game, injured his left hand in the first half and he didn't play in the second half. Sima didn't score but he had five rebounds and two blocks in 11 minutes.
Xavier: With the victory Mack tied the late Skip Prosser for second place on Xavier's all-time wins list with 148. Pete Gillen is the leader with 202. ''Skip meant a lot to me. I'm not half the coach he was and I mean that with all sincerity,'' Mack said. ... This was the second game the Musketeers had to play without freshman guard Edmond Sumners who is still in concussion protocol after falling hard to the court early in the loss to Villanova. ... Coming into the game Xavier had four players in the top 15 in the Big East in free throw shooting. Davis leads the conference at 93.9 percent. Davis went 9 for 9 and the Musketeers were 22 of 29.
St. John's: The last time St. John's beat a top 10 opponent in Carnesecca Arena was a 76-74 victory over No. 9 Pittsburgh on Feb. 28, 1987. The building was called Alumni Hall then... The first college game for Mullin was against Xavier on Nov. 27, 1981. The then-Redmen won 75-55 and Mullin had 10 points, four steals and two assists in 19 minutes to start his Hall of Fame career. ... Naismith Memorial Hall of Fame coach Lou Carnesecca was at the game, two days after his 91st birthday.
Xavier hosts DePaul on Tuesday.
St. John's is at Marquette on Saturday.