A method of implementing challenges in single-player games starts with a server receiving a challenge from a first mobile device and transmitting the challenge to a second mobile device. The challenge may include a high score or an achievement of a single-player game obtained on the first mobile device, The server may determine that a new high score is obtained on the second mobile device that is equal or greater than the high score included in the challenge or that the achievement is completed on the second mobile device. The server may then transmit a notification to the first mobile device that the challenge has been completed. If the server determines that the new high score is obtained, the notification may include a new challenge from the second mobile device to the first mobile device and the new challenge may include the new high score. Other embodiments are also described.